The voice acting is indeed terrible, worse still is the fact that they blew a crapload of money on getting Patrick Stewart and that guy who played Zod in Superman 2, Sean Bean was great though. The complete lack of any dialogue in favour of NPC monologues at the player is very grating, that was a major flaw in Morrowind and they've only made it worse with the full voice acting.
Landmass is only slightly better than Morrowind (16 square miles compared to ~10 square miles according to Bethesda) but pretty pitiable compared to the second game in the series Daggerfall (~180,000 square miles - yes there are 4 zeros there)
Not to mention the complete loss of any real reaction to anything you do, no 'roleplaying' by the player has much substantial effect in the way the world plays out, the stereotypical us-vs-them main quest and the complete rewrite of Oblivion and Mehrunes Dagon into being hell/the devil respectively, combined with the depiction of Oblivion as something a 15 year old would draw if asked for a depiction of hell.
Other than that, yeah it's okay I guess, just not able to hold a candle to Daggerfall. Morrowind at least had some moral ambiguity in the storyline which made it lovablke despite it's numerous gameplay flaws; Oblivion goes the whole hog of stripping out the most worthwhile and original RPG elements that Daggerfall pioneered.
It sold well though, and in the end that's what matters to Bethesda.
