It's mainly the maintenance that gets to me after a while. I don't mind the dungeon crawling, fighting stuff is still fun and, if you don't use your best skills, still a bit of a challenge sometimes at higher levels. Looting gets increasingly boring and tedious though, but I've more and more phased out the less expensive items and leave a lot more behind.
But what gets to me is the stuff that needs to be done between dungeon crawling. I need to get back to my house, wait/sleep until it is day so I can haul over the next wheelbarrows full of loot to the appropriate merchants, get all my ingredients in a chest, pick up the enchanted crafting items, go make potions, sort potions, sell potions, enchant away my new hoard of soul stones, sell enchanted items, see whether I can upgrade my armor again, same for weapon, see whether Lydia can get something better, upgrade her shit, store all the enchanted crating items again, store all the other crafting equipment, find some quests to do.
And then I can go crawl another dungeon, perhaps 3-4 of them, before the whole shlebang hits me again and the game sulkily demands another half-hour/hour of maintenance again just to keep all the crap piling up on me stored/sold away and to keep my skills going up.
So I think I'm going to put the game to a rest for now. I've barely scratched the main quest line, only done the companions quests, and pretty much none of the other guilds/colleges. I think I've levelled far too quickly, already at 48.