I played it vanilla the first time as well. I bought it mostly because I was still playing on a crappy computer and I could actually run Oblivion with settings turned down... and I had no internet access at the time.Serris said:oblivion sold pretty well, despite one of the easiest to crack DRM (i seem to remember i didn't have to put it my dvd to play).
and i'm pretty sure not everyone installed a lot of mods. a lot of people bought it, played it, liked it, moved on.
to clarify though: i did use mods. but only long after playing everything there was to be had with vanilla.
The biggest issue, the issue that almost everyone cited being reason for hating the game, was the fact that everything leveled with the PC. You'd wander into some little rathole at higher levels and be fighting Goblins using Glass and Daedric armor and weapons. Pretty silly.
Honestly, the core game is actually pretty much just fine (unlike FO3) - most of the mods I use just make the game look nicer, increase stability, and generally rebalance combat a bit. Oblivion doesn't need near as many overhaul mods as does Fallout 3; that's probably because Oblivion didn't effectively shit on the TES character creation and development system like FO3 did for the SPECIAL system.