I'd probably give vanilla oblivion 5/10.
Overall presentation is passable, nice graphics barring low res textures, nice music however few tracks, pitiful voice acting and bad sfx, despite the nice flora and fauna the world itself is largely generic computer generated terrain, no natural features like valleys, plateaus, rivers etc. lots of pointless and unrealistic 2 house settlements scattered about, loads of almost identical dungeons/forts/ruins every 10metres. The user interface was terrible, a huge step back from Morrowind which wasn't exactly the pinnacle of design itself, clearly design (badly) for a console, the font was massive and what appeared on one screen in Morrowind eg. the inventory & map, was spread across many tabs which resulted in constant tab switching, infuriating!
The need to travel the environment and subsequent exploration was reduced by the fast travel system, though many complained about this, due to the stupidly slow movement in the game it was necessary at times, in reality horses can travel in excess of 50mph for short distances (Oblivion is only 4 miles across) but in Oblivion it felt more like 20mph.
The AI was terrible, you could kill an enemy and the other enemies would just stand still and not take cover or even really take any notice.
Combat system is almost non existent, hold right mouse button and continuously press left button to attack, no skill needed, no gore or character deformation whatsoever so killing is not very satisfying. Magic is largely useless, tons of useless tat (this is a common theme in the Elder scrolls) spells like burden 10points for 10 seconds, spells use way too much magicka, and require a stupidly high skill level to use, visual effects are mediocre.
Scaling system means game gets harder as you level up and you can complete the game at level 1, no character development required which defeats the point of an rpg... bandits have glass armour which is completely unrealistic, and loot scales so at level 1 the 1,000 chests every cave seems to have are filled with paintbrushes etc. and at level 20 every chest has some stupidly valuable items in it. Levelling system is terrible have to choose major skills based on which ones you won't use in order to get the multipliers, defeating the concept of major skills in the first place.
Merchants don't have any decent weapons and armour at the start of the game, so there's nothing to aspire to, no reason to get any gold and explore, no reason to have a thief character because armour protection is based on armour skill, thus glass armour has the same level of protection as leather so there's no reason to steal it.
Terrible game engine results in stutter and long loading times, and there's over 1000 unfixed bugs. Inadequate modding provisions requires a number of tools such as OBMM and wyre bash to be used.
Overall the game feels hollow, like butter scraped over too much bread.