I never had the benefit of playing the predecessors of Oblivion, but I absolutely love the idea of playing as a werewolf, moreso than a vampire, as I'm a bag fan of the feral nature etc.
While I would be happy with simple inclusion of these two 'afflictions', It would be a really nice touch to have some faction-specific quests for each. Nothing major, as it would take away from the people who didn't fancy being cursed, but something which makes the affliction somewhat worth it, beyond the abilities gained.
From what I've read, werewolves have been done pretty well in this series already, especially with the inclusion of Hircine's ring.
If you really want to play an awesome game as a werewolf Mickey, you should probably get your hands on the Morrowind Game Of The Year Edition. Hopefully your playing p.c., if so, there are a boat-load of awesome mods (including werewolf mods) that you should look into.
But I do agree with you in regard to have faction-based quests in regards for both werewolves and vampires. This idea would make a really great expansion pack...