I picked this game up in 2007. Stuck with it throughout the years. I've seen the Wings expansion's completion, the gradual raising of the level cap from 75 to 99, the rage some felt when SE added the Empyrean and Aeonic weapons, and much more. Even if I put it down to focus on my own life for a while, XI is that one MMO that calls me back.
What to expect: XI is a game in and of it's time. I'd suggest looking up Kwech's installation and login guide to newcomers. The video thumbnail says "Beating the first boss"... and yes, setting the account up is the first boss. Once you've beaten that, pick your server. Servers aren't region or time zone specific. All the servers are in Tokyo, Japan. Yet even then, some servers are known to attract majority English or Japanese players, two prime examples being Bahamut and Odin. This also means the servers themselves are beholden to JST. If something you need to do says "Once per conquest tally", it means you need to wait untill 10am on a Monday morning on the U S east coast to get that done. Meaning expect the game to ask you to commit time a d patience.
XI will also not hold your hand. The closest it comes is certain Records of Eminence quests which can only point you in a general direction. Your best friends are now BGWiki, FFXIClopedia, and FFXIAH. These three sites contain XI's aggregated knowledge, shared by those who've done what you're about to do in the game.
SE has also listened to complaints that XI can be too hard and unforgiving for a fresh out of the box new player. They've since added Trust NPCs, a Mount system, the Records quests I mentioned earlier, and a load of other fixes. The game is much easier to get into and get established now because of these changes.