I'm currently running an assload of mods. My Ungifted mod (on my fighter), Arriere's Steel Weapons & armour mods, Darker dungeons, Darker nights, Deadlier traps, Faster Arrows (150%), Arcane Velocity (150%), the guild ranking system that only lets you loot certain things at certain advancement levels, the creature behaviour modifier that makes certain creatures more aggressive, or use shields and other abilities more often, a mod that lowers the amount of damage your weapons & armour take during combat (seeing as I'm using my weapons and armour more often), BTMod (extended interface), and the coloured map mod.
I'm pretty sure that I'm using a few more, but I can't remember all of them. I bet that a few of these mods probably clash with one another. I think that I'm eventually going to just put all of my favourite things into the one mod/modpack and fix up the relationship problems in whichever way I prefer it to be.
I know that this seems like I am changing the game in a rather fundamental way, but almost every mod I am using makes the game harder, especially my Ungifted mod and the two arrow & spell speed mods.
Archers and Mages are now straight up deadly. At level 1 you can kill someone in three to four bow shots depending on their armour level, but at the same time every other archer can hurt you just as badly. Add the fact that I can't heal myself without potions and everything suddenly seems Really Fucking Scary. Arrows and fireballs are now very hard to dodge, as magic should be. I was getting really really sick of lazily sidestepping fiery balls of doom, then nailling the mage with an arrow or two because he was too stupid to move. This helps a lot & makes the game more challenging.
I'm thinking of adding in the "random Oblivion" mod so that things are not levelled perfectly for the character anymore, also. When you can walk around and know that everything is pretty much your level it kind of takes away some of the surprise of a real RPG. I want to occasionally turn a corner and find someone who I have to fight hard to kill, or perhaps that I have to run away from until I am a bit tougher. I'm not 100% on how it works though. If it seems like fun then I'll probably give it a go.