Everything.

I want to be able to log in at the level cap and wonder which of the multitude of things to do I wanted to try that day. FFXI was great for this; between Dynamis, Limbus, Assault, Salvage, Nyzul Isle, Campaign, and HNMs of various kinds there was always something different to do with groups of different sizes, with different time commitments and different reward systems. I think ideally FFXIV should have dungeons of various sizes (light party, full party, alliance/raid), huge HNMs roaming around, long quest chains and point systems that allow even soloists and casual players to slowly obtain endgame-quality stuff...and for all of these different themes and "flow" options. For instance, just within one part-size dungeon you could have straight fights against enemies, or something like we have in Toto-Rak now (or how Limbus was) where you get to choose your path and rewards.

Basically what we need, above any one thing any group of people think is the "best" type of content, is options.

The only thing I would prefer was kept out of endgame would be storyline. I don't think main story quests should necessarily be easy, but I think the majority of players should be able to witness all the cutscenes in the game with some effort, whether they have a huge raiding LS or not. Some side-stories like the one in FFXI's Dynamis are fine, but the "main" stories should be left for regular play, at lower levels and for smaller groups than raids.