I respectfully disagree with the OP. While I agree that the flexibility to use abilities across classes makes for squishy definitions, that isn't all bad. My three main jobs in FFXI were WHM, BLM, and SMN. WHM was a great party job but had trouble soloing EPs. SMN was a great solo job but only so-so in party play. BLM was great sometimes and horrible others. None of these jobs were welcome in TP burns.
With the way classes work now, there is no reason any class can't solo, and then with the switch of some abilities fill a niche in a party. When I am soloing on GLA, I equip some spells and unequip Provoke and Cover. When I solo on CON, I equip Shield Bash and Phalanx instead of spells.
I wouldn't mind seeing a little more differentiation between classes, but I like the ability to configure a class as the situation warrants. I would not want to return to the situation in FFXI that if you didn't have the right job, you would never get a party invite.
I agree that flexibility is nice, but every game I've played since FFXI's release has had several modes for their classes, and at this point, they are all very easy to transition between. WoW has dual talent specs, Guild wars allows easy respecing (presumably GW2 will aswell), DCU has both DPS and other modes for their classes. My point is that defined classes and flexible playing styles aren't mutually exclusive anymore.
Now while I will admit that FFXIV is more flexible (once you've put in the time to level everything you need), I can definitely identify with the OP's missing specific class identities and roles.