My first one was a khajiit assassin wannabe, I spent the first 6-8 levels sneaking around Whiterun stealing anything movable. I sucked at sneaking when I actually tried to kill someone, almost all bandit fort visits ended up with either me running away, or after the first failed sneak attack attempt I just rushed in and slashed everyone into pieces. At around lvl12 I got a better grip on the game, restarted and successfully pulled off the whole assassin thing :D

Sabrecats, cave bears and the most annoying are ice wraiths because of their wonky animations and bad visibility. Maybe the Thalmor as well, when a group of three attacks me at lower levels I usually get lightning bolt spammed to death.

Oh god that makes rented rooms so immersive I can almost feel my head touching the pillow when my character sleeps :P Have you tried the HD silverware mod? I can't enter the jarl's palace without that installed, it's an all time must have for anyone who wants the best Skyrim experience :D

How could I forget this, Skyrim feels bland and empty without this mod. It truly is game changing 

SKSE (duh), Frostfall, RND, SkyUI, iHUD, Wet and Cold, Immersive Armors, Dual Sheath Redux, Duel Combat Realism v5, Winter is coming - Cloaks, the Unofficial Patches. These are the most important mods for me. I could also mention the recently taken down Climate Overhaul - Winter mod. Paired with Frostfall it makes Skyrim an even harsher place, and since it completely changes the areas that weren't covered in snow before, it makes Skyrim feel fresh. Oh, and due to the amount of times I've been killmoved at high difficulties Violens has became an essential mod for me (you can finally toggle Player killmove immunity). As for graphical mods I love Kinematic Enb and ELFX combined (interior module only), and also most of the popular high-res retexture mods (Skyrim 2K, Amidianborn, SMIM etc.)