Favorite: The combat feels satisfying to me, and the visuals are very pretty.
Least Favorite: Two big ones for me. The first is that I feel that magic is underpowered, but leave that aside for a moment. Magic has been so chopped down from the removal of spells and schools of magic, and the removal of spell-making and the lack of a 'Mage' infrastructure (College of Winterhold vs. a Mages Guild in most major cities) really detract from playing as a magic user. I have no real desire to play one in Skyrim.
The second thing is the lack of special things to do in general. I want more Guilds - two or even three times the Guilds we have now, with two or three times the quests. They are
so important and yet so under-utilized and under-cared for in Skyrim, compared to Oblivion and certainly Morrowind. In Skyrim they're either too short like the guilds, or not well-done like the Stormcloak/Imperials. I feel like so much of my game is just wandering around looking for random quests to do to try and space out doing the main quest or just add some bulk to my playthrough.
Every magic character basically has to join the College of Winterhold. Every warrior character basically has to join the Companions if they want to do a guild, and it gets a bit old doing the same seven quests over and over again each playthrough.