I think WoW appeals to most people because of the easy and very casual play. FFXI appeals to the opposite type of person the most. It does take a lot more work to get anything done and FFXI lacks the type of PvP that people prefer from WoW. However, in FFXI you definitely feel like you've accomplished something when you hit 75 or finish certain missions or other similar things.

I've known a good number of FFXI players that have tried WoW. Some stay because they are angry with the customer service in FFXI, but I know quite a few of them quit after 3-4 months. In that time they've often leveled 2 or 3 toons to 60. Endgame options are somewhat limited in WoW compared to FFXI and there's not the same sense of community.

You can solo to 60 fairly quickly and very casually. Then you do instances over and over and over... or PvP a lot. Long time FFXI players find it limiting and boring. If casual gaming is your thing stay away from FFXI. FFXI has a ton to offer both mid level and especially endgame. You retain your character and can level several jobs on one character instead of having to level other toons.

However, FFXI does have a sharp learning curve and is very difficult to start at first.