Delayed response FTW! Obviouslly no one else is gonna tackle these questions, at least not here, so I'll answer even though I doubt you're still checking back. ;3
--It's definitely different. It's still heavily party-based, almost to the point of annoyance if you don't have friends/linkshells to rely on, and the ATB/turn-based stuff is pretty much gone. The overall feel is definitely Final Fantasy, but you actually have to pay attention to the storylines to get the feeling.
--Apparently, FFXI boasts a 50,000+ subscriber base, last time I saw anything about it. (And I think that was in an ad/announcement for the new expansion)
--Every time I've "/sea all" 'ed, I've gotten anywhere from 1500-3000 people online at a time on the Cererus server, and I assume other servers are similar. I'd say, on average, about 2200 people would be on during a good day and at normal times.
--Answered.
--I've been playing since December of '03. Well, I say playing... I've had an active subscription since then. I have only really been playing on-and-off. I have a lot of my MMO and online gaming roots in this game. It's pretty much the one game I just can't let go of, can't say for sure if it's because I like it that much or just because it's had that much of an impact.
--Basically, you solo from level 1 until about 12 to 15, for some classes 20. After that, except for VERY few classes, you'll be LFG for the rest of your levels. At level 18, you get the ability to use a Subjob, which essentially enhances your stats and abilities with the abilities of another class, which alwasy presents itself as half the main job's level. (I.E., if you're a level 20 Warrior and a level 10 Thief, you could be a level 20 Warrior/10 Thief, or a level 10 Thief/5 Warrior.) At 30, the game gets a lot more fun because you gain access to advanced job quests, which open up a lot more options when playing the game. Personally, I don't like any of the primary jobs nearly as much as I like some of the advanced jobs, but that's personal preference. The game is highly socially-based; you can't do anything without a party, essentially. If you can make friends and get into some linkshells, you should be fine.
If you have any other specific questions, feel free to PM me. (Assuming anyone even checks back on this. =p)