I used to play FFXI and loved it, but quit in 2009 when my old LS went bust. Now I play EVE online.
For me an MMO is about community.
I was introduced to FFXI by a friend. FFXI was my first MMO and I remember how excited I was to be playing amongst, and interacting with, other people around the world. I remember showing the game to some other friends a few years later and they were amazed at the concept of chatting to some random guy form another part of the world (they lived a sheltered life, but are now much happier). I think that is something that is taken for granted nowadays and people are MMOs are growing more secluded with people playing solo with no need to communicate with one another.
After I quit FFXI I tried a few MMO's but nothing that I really felt a part of like I did in FFXI. That was until I started paying EVE online.
Like FFXI, EVE is really a game that (in my opinion) you get little out of unless you are art of a community. I did some research before I started playing and joined EVE University - a huge corporation dedicated to introducing new people to the game. I met some great people there and, after about a year pottering about and exploring the vastly different play styles on offer, I joined a PVP corp based in the most hostile areas of space.
I can honestly say (and I'm a little embarrassed to say) that I have never had such an adrenalin rush from a game as I had in my first few encounters in EVE PVP. This is something that has continued and I love nothing more than getting together with a bunch of like minded people (there are 400,000 subscribers to choose from - all based on one server) and hunting down unsuspecting others, suspecting others or even being hunted by and blown up by others - all for fun and the pleasure of competition.
I think EVE is somewhat of a unique concept amongst MMO's these days - the emphasis is on whatever the players want - there are so many options and it's the players that provide most of the content rather than the developers. My main is in a dedicated PVP corp, but I have a couple of alts in a freighter corp that earns money providing a removal service for other players. Both corps are entirely player driven and I would expect that anyone who had played the game for some time would have heard of both of them.
Of course, you can also play casually. For example, if PVP is your thing, you can join up with RVB (red vs blue) - two corporations instantly at war with one another so you can log in and get a fight within minutes. Or, if you prefer, you can join up with any number corps specialising in industry, faction warfare, PVE etc.
EVE is definately a game you need to be a part of to get the most out of. Absorbing yourself within a group of other people and achieving things together is, in my opinion, where the game truly excels over others. There is a steep learning curve and a lot of patience required to get the most out of the experience, which is where it falls down for a lot of people.
In essence, if you're willing to put in the time and effort, then I think it's a unique gaming experience, but its certainly not for everyone.
I can't help wondering if MMOs are destined for specialisation. I don't think that companies can afford to keep on following out WOW clones in the hope of being the next WOW - there will never be another WOW. I think companies need to start looking at niche markets for MMO's and learn to expect lower subscriber rates - you simply can't please everyone, but if you can please even a minority than you can still make a decent profit as EVE has shown.
Salt and corrosion... the infamous old enemies of the crime fighter!