Do you mean the actual quality of the spoken words (the "acting" aspect of it) or the fact that only a coupdl voice actors do the voices for all the races.
For me, although the quality is spotty (the Nord/Orc Female voice actor is wretched, but the Imperial male voice actor is pretty good), having so few voice actors voice so many npcs really detracts from the whole voice acting aspect of the game. I don't think every single NPC needs a seperate voice actor, but to have thousands of NPCS done by 5 or 6 voice actors is a bit less than impressive.
<{POST_SNAPBACK}>
Eh...both, really. I'm sure I missed a BUNCH of content in Oblivion, but all the dialogue really never filled me with anything other than a means to an end, like in NWN. What killed me most about the number of actors was that I can deal with having one person for every race (I suppose), but to have dark elves and wood elves sound the same and have the same 'flighty' attitudes? I mean, come on.