What about how old DnD games used to do mixed classes? That might be cool to see. Does the DnD MMO have them?
So you could have some flexibility but not as much as Rift apparently has.
Caveat: I have not played DDO in a year and a half.
DDO allows you to mullti-class up to three classes, loosely based on the 3.5 rule set. Alignment restrictions apply, but there is no racially preferred class system, or any experience point cap for the additional classes - you get 100% XP, no matter how many classes you have.
Through a combination of classes, abilities, and the right gear, you could manage a lot of variety. There were a number of Wizards who had just enough spells to buff themselves for melee, Rogues designed to tank, Bards as primary healers, Battle-clerics. It was also possible to severely gimp yourself by making poor combinations.
I thought creative multi-classing brought a lot of great flavor to the game.
I haven't played Rift, but I see some benefits, compared to DDO. The ability to switch roles means you needn't spend forever trying to build group with just the right class combination. If they buff/nerf some class, you aren't required to completely re-roll and level up from 1, even if you might have to make drastic changes, it sounds like you should still be able to re-tool it into a useful Rift avatar. For a casual player like myself, I like the idea of having a main character that I can play in multiple roles, for variety, rather than leveling up multiple alts for the same purpose. Plus, DDO had some quests where, say, a Rogue was absolutely useless, so it was difficult to find a group that would take you, in order to get credit/reward for completing it. Mind you, a good guild would support you and take you through it, but a bad PuG would pass you over for something they considered more practical. On the other hand, the table top gamer in me liked the idea that classes were somewhat unique, and would be differently useful - and more or less useful - in different situations.
Prior to that, I played Asheron's Call 2, which world I loved. I don't know how it compares to the original Asheron's Call which is oft-cited in this thread, but it sounds like I can get some of those old feelings back in Rift. I also tried Everquest 2, at one point, and found it to be rather dull.