If clerics of different deities do lack any real distinct flavour in terms of actual abilities (powers, prayers, rituals, dodads, whatever) then I'm going to be very disappointed. Real differentiation between characters that should be as different as chalk and cheese is something I think is really important to the game. But then I also wanted Complete Mage to have each of the specialist wizards as different classes, so I can't say that I was truly expecting this differentiation.
But WOTC if you're listening - I'm one of those that would definitely pay for the book that gives me this very tangible distinction. A priest of the god of Death and Decay should have VERY different abilities from priests of the gods of Righteous Duty, The Sun, or Fluffy Bunnies.
For clerics this doesn't need separate classes, just separate trees of powers - like Kzach mentioned. As long as there is a broad enough array of powers to support this I'm happy - and if need be I can then rule that players of god X can only take specific powers. But if the only difference between Morgroth of Bane and Kylie of Fluffylepus is the way they dress, I may have to go all emo
For specialist mages I'm not sure, maybe it could also be done that way or maybe it requires separate powers - but I want the Necromancer to feel dramatically different from the Enchanter and the Evoker. Maybe this can just be done by careful selection of the available powers - I'll wait for the books to see if that's the case.
In either case if the core doesn't offer it, I'd certainly buy the supplement that gave me the options.