These mods are fixes that Bethesda should really have included by now, and super-useful (but lore-friendly and not game-easing) mods that EVERYONE should have. This list is VERY conservative, so it's short an sweet and is probably missing a lot of mods you might enjoy. There are a few that cannot or should not be acquired on Steam and can be acquired the Skyrim Nexus using the Nexus Mod Manager:

SkyUI
Convenient Horses
Extensible Follower Framework
HighResTexturePack DLC Tweaks (http://skyrim.nexusmods.com/mods/14719)
HD Textures DLC Fix (http://skyrim.nexusmods.com/mods/8236)

Notes:

***Extensible Follower Framework: *DO NOT* level your followers past their level caps and *DO NOT* use more than one follower. Never use the "Train" command, because Bethesda fixed the original bug where followers didn't level AT ALL. Raising your followers past their level caps makes the game even easier at higher levels, and it's already easy enough on Master difficulty. Recruiting more than one follower at a time also makes it easier. Resist the temptation... it's not like they banter with each other, anyway.

***Convenient Horses: Do not use the tweaks that increase the horse's speed, stamina, or health. Do not abuse the horse inventory feature to carry lots of loot to sell.