The AI of Oblivion was terrible, they just charge straight at you like zombies and after a while all the attack styles became so tedious and similar, that it really felt there was little difference in taking on a Dreugh or a Troll or an Ogre.

I see they are putting Giant Spiders in, this pleases me as I have a thing about Giant Spiders. What does worry me though is how they've animated it and used the AI. I'd hate to see a Giant Spider that just runs at you in a straight line and does the same old jump attack of a Dreugh.

I would like to see more variation in attack styles and decisions during combat.

Agreed. 100%

I mostly hope they've improved the AI and LOD so there can be massive battles.

...but why I have the feeling that's not the case, and my hopes are slowly dying off?

In the article it mentions different behaviours of different monsters. This is at least a minor AI improvement.