This an updated and fancied up version of my original 'So You Just Hit Level 50 in Patch 2.1' post from a few months back. Now all up to date for 2.2 : ) I've condensed some of the earlier steps in light of the content being eased, and added in the 2.2 content.

So you just hit 50, and now you're wondering where to go from here. Well, I don't think anyone can tell you explicitly what should be done in exactly what order, and you certainly can skips steps, but here's the most gentle slope for introduction to end-game.

1. Fresh 50? Ready to see what the world has to offer? Lets get to it then.

   • Finish Your Job/Story Quests - Your Job quests will net you your last skills and your Artifact gear. Do them. The main quest up through Praetorium is required for practically all other Level 50 content.
   • Replace Your Trash Items - If you have any items below iLevel 45, they should probably be replaced with crafted/purchased items. You should pay particular attention to your weaponry.
   • Castrum Meridianum and The Praetorium - You'll need to run them once for story. If you feel uncomfortable with 8 man content you can run them once or twice extra, but you should probably avoid farming them outside of Duty Roulette: Story Mode, which you should now have unlocked, as the base Mythology reward is quite low, and they provide no gear.
   • Wanderer's Palace and Amdapor Keep - You'll have to do these anyways for your GC and Relic quest eventually anyways, but they also drop iLevel 55 gear to get you started, and are an easy completion even in fresh 50 gear (particularly now that AK's difficulty has been nerfed); they should help ease you into the idea of 4 man dungeons at level 50, and how various classes perform.
   • Relic Quest Line - It's probably a good time to start on your Relic Quest. It'll take you through Chimera, Hydra, and back through the content you've completed at this point. At minimum you'll want it all caught up before you get to Titan some time down the road.
   • Ifrit Hard Mode - Unlocked in the Waking Sands, he's an easy and simple mechanics fight which drops iLevel 60 Weaponry.
   • Some Things You Might Miss - The story isn't over after Praetorium: the original 2.0 story ends there, but 2.1 and 2.2 extended the story after that point. Check in with Minfilia at the Waking Sands to continue on from there. As a level 50, you also now have access to the Glamour unlock quest (in Rev. Toll), and the Hildebrand questline (starting from Wymond in Ul'dah). There's also lots of other unlock quests now available to you now for various bits of content, so grab any level 50 quests you see. You should probably also begin leveling your sub-classes to get any cross-class skills you're missing, though they're not of crucial importance for a little while at least.
   • Want to Skip Some Steps? - Purchasing i70 (or even i90) gear on the market boards, or tackling the next sets of 4 man dungeons can help catapult you forward quite a few iLevels, and reduce the amount of grinding you'll need to do. You will probably still want to clear your relic quest eventually though.

2. Ok, so you made it through the basic content, now it's time for a few fights with a little more mechanical complexity, and better rewards.

   • Garuda Hard Mode - Like before, unlocked in the Waking Sands, Garuda is a more complex fight, and one which requires some gear at least to pass various checks on your stats. She turns out cool looking iLevel 70 weapons for your trouble.
   • Titan Hard Mode - The last barrier in the route to Relic, clearing Titan should be your goal for this tier. The first fight that will really stress both mechanics and raw numbers for many players. An HP in the range of 3200+ is traditionally considered the minimum for this fight. Also drops iLevel 80 Weaponry. As a note, having Titan HM cleared is a requirement for access to The Binding Coil of Bahamut, when you eventually get there.
   • Thornmarch Hard Mode - Feeling the pain on Titan? Push on through the main post-Praetorium story and eventually you'll run into this funny looking pile of moogles. It's a fun fight with a very low challenge level (due to it being a story fight) that drops iLevel 75 Weapons. A helpful stopgap if you find yourself rockblocked by Titan for an extended period of time. Or if you just really like Moogles.
   • Copperbell HM, Haukke HM, and Pharos Sirius - The next step up in 4 Man dungeons, these guys provide a little more challenge, but better rewards, in the form of iLevel 60 gear of all varieties. Haukke Manor HM is also reasonably effective Myth grinding if you want to rack up those tomes, but don't want to jump into the harder 4 man dungeons yet.
   • Crystal Tower - If you can punch your way through the tiresome unlock quest, Crystal Tower can be a nice source of easy iLevel 80 gear, particularly now that they've removed the loot restritions. Just be aware Queue times can get long sometimes and there are often at least a few dim bulbs in any 24-man alliance.
   • Myth Tome Gear - By this point, you're probably sitting on at least a couple hundred Mythology Tomestones. Take those over to Revenant's Toll and you can exchange them for iLevel 90 Job-Specific gear, or just turn them into materials for a bit of extra gil.
   • Some Things You Might Miss - While they provide no gear rewards, Leviathan HM and Gilgamesh are two pieces of content you may want to get out of the way. The first will be needed for access to Leviathan Extreme, and both are needed to unlock the Duty Roulette: Trials option, which is convenient for Soldierity farming, or just for fun. Also, don't forget, if you do all the 4 man dungeons I've listed so far at least once, you should now have Duty Roulette: High-Level unlocked.
   • Want to Skip Some Steps? - Filling out any missing i70 slots via the market board, or jumping ahead and farming the Expert class dungeons can help boost your iLevel the last few steps needed to jump into some of the upper content levels. As before, i90 Gear via Myth or Market Board is also highly helpful.

3. Got your iLevel floating somewhere around 70? You're just about ready for the first steps into End-Game, but first, a few stop-gap items of interest, that may help you push to the i75-80 range, since the last tier was getting a bit too long, and you will probably want something a bit more mean than full i70 going forward.

   • Brayflox HM, Halatali HM, and Amdapor Township - The Expert class dungeons, these are the hardest 4 man content currently (still pretty simple stuff), and are perfect for farming yourself some Soldiery Tomes and i70 gear. They even drop crafting materials that can turn you some quick gil too. Clearing all three unlocks Duty Roulette: Expert.
   • Ballad: Ultima - The Ultima fight is not freebie; it has a difficulty level similar to that of Titan HM. Still, for the adventurous with gear in the iLevel 65+ range, he can be taken down for some solid iLevel 80 Accessories.
   • Relic Zenith/Atma/Animus - Save up those 900 Myths over time and upgrade your Relic at Gerolt's forge for a quick 10 iLevel bonus in stats. Once you've done that, you can start in on your quest to further upgrade it to a Relic Atma/Animus, if that's your kind of thing. Be aware it takes an exceptional amount of time to fully upgrade a relic, and (at least currently) they are not a Best-In-Slot weapon. The advantage being you don't need upper level content clears to do it.
   • Soldiery Tome Gear - At this point you should have some Soldiery tomes stacked up, if not a whole lot of them. If you have enough, you could purchase yourself some i100 gear with them, from Rowena, as usual.
   • Cross Class Skills - While some may have them all sorted by now, going forward into the real end-game content, you should really take care of any major lingering Cross-Class skills you may be missing for your job, such as Invigorate/Blood for Blood, Raging/Quelling Strikes, Swiftcast, Virus/Eye for an Eye, and Provoke.
   • Want to Skip Some Steps? - At this point, there's not a lot of skipping to be done. The only thing left you can do is buy yourself some i90 gear off the Market Board, but they come priced at a pretty penny, due to the difficulty of making them and requirement for Myth Tome materials.

4. Got your Relic? All Pimped out in iLevel 70-90 gear? Time for your first foray into the big stuff.

   • The Binding Coil of Bahamut - A Five part dungeon with high mechanical complexity and numbers (for your iLevel). Find yourself a good PUG or Static and have it. Drops iLevel 90 gear, with iLevel 95 Weapons as a reward for Turn 5. Currently you cannot solo-queue for Coil, but that is set to change (along with adding Echo buffs and miscellaneous nerfs) in the coming weeks. Completion of all turns is required for access to The Second Coil of Bahamut (colloquially termed 'T6-9').
   • Garuda, Titan, and Ifrit Extreme - An alternative option to a Relic +1, and a nice way to pick up some i90 accessories. Clearing all 3 of these will get you a free iLevel 90 weapon of your choice, and is required to access the next two Extreme Primals. Also drops mounts, furniture materials, and other fun things.

5. Got the above done? At iLevel 85-90? Time for the top-class stuff.

   • The Second Coil of Bahamut - A Four part dungeon with yet more mechanics and even higher numbers. This is the hardest content in the game currently, with i110 rewards, and i115 weapons for clearing the final turn.
   • Leviathan Extreme - The real version of the wimpy Hard Mode Leviathan you probably completed ages ago at this point. A mixture of dodges, DPS checks, and coordination checks, this guy dishes out i95 weapons which can be upgraded to i100, and a number of other fun non-gear items.
   • Thornmarch Extreme - Once again, welcome to the real version of something that was previously a walk in the park. A fight with some strange mechanics and numbers, hold on to something if you want to snatch an i100 neck piece from these moogles before they sweep you away in a tide of terrifying fluff. Like Levi EX, this also drops a number of fun non-gear items.
   • Upgrading Soldiery Gear - That i100 gear you got from Rowena's minions can be upgraded to i110. In order to do so, though, you'll need to get some Sand/Oil of Time from the Second Coil of Bahamut. Alternatively, if you find yourself with an Unidentified Allagan Tomestone from the same, you can toss down 1300 Soldiery to turn that bad boy into an i100 weapon of your choice, which can then be upgraded with Sands of Time to i110.

Hope that helps folks, and let me know if you've got some questions or comments, or if I missed anything. :D