Ummmm... By focusing on the plot and each quest individually. Unexpected, right?
Well, I sincerely hoped that there was someone else in the game besides assassins. But if all the NPCs in the game are assassins, I'll retract my claims.
Based on Ar's post, I assumed he also meant the series of assassin quests, because, in my opinion, it's a very good part of Oblivion, if not one of the best.
Well, okay. There's also the Thieves Guild, where there were no killings (as a goal:rolleyes:). Or will you now write "steal, steal, steal"? It seems to be out of helplessness:rolleyes:
The plot development is quite good in Morrowind as well. The plot missions were sometimes stupid, but the overall plot line is quite decent. In any case, it's subjective.
In general, in my opinion, the RPG genre is one where the best is yet to come, and largely due to technological advancements (yes, yes, that's exactly it, good graphics are very important here). The Witcher has demonstrated this quite well, combining a non-linear (or rather, branching) plot, good graphics, decent physics, a skill and spell system, etc.

Барон "Vampires"
Отдай свои прошлые сны,
В сомненьях себя обрети,
В бешеном танце душа кружится!
Не думай, что будет потом,
Здесь и сейчас мы живем,
Стань хоть на миг сам собой, слышишь!