World/ dungeon design is great, variation is also great and on Morrowind/ Oblivion level, granted Morrowind had way more diverse architecture. Level scaling is pretty ok, similar with Fallout 3.
Dungeon design is rather terrible. There are way too many "if you go past this point, you can't go back unless you beat the enemy" situations, which serve no purpose but to make it a hell of a lot harder for modders to tamper with the level scaling. And while it's quite practical that every dungeon of any size has a convenient shortcut back to the entrance, it's also hugely repetitive and hugely unrealistic. But of course it's a natural consequence of the removal of teleportation spells. When you can't cast a mark at the beginning of a dungeon and then recall back to it once you've cleared the dungeon, you're stuck with a lot of long and boring treks through empty ruins. Which is why BGS made every single dungeon out there conveniently have a shortcut to the exit that can only be reached from one way.
The dungeons themselves are mostly fine, though they suffer at higher levels because you've seen it all. Every dungeon is level scaled so every dungeon has a completely predictable difficulty. Since they're scaled to be just right for your level, it means that if you end up powerful relative to the scaling, then every single dungeon in all of Skyrim will be a cakewalk and there won't be any dungeon out there that actually is dangerous.
My main character is a level 50+ warrior Breton. You can probably guess how much danger she's in when she goes dungeon-crawling. In case you can't, the answer is absolutely none whatsoever. Dragr deathlords haven't been the least bit scary for 20 levels or so, Forsworn can be a problem in groups but since they're rarely more than two at a time, they die easily. Bandits? Pffft. Weak, pathetic fools. And this is a general problem with excessive level-scaling of enemies. It easily makes all the encounters feel the same and it means that there aren't any dungeons where I should feel the least bit scared of entering. There are no "dangerous" areas that are relatively hazardous and no regular zones which pose a challenge but are generally safe, as long as you're patient and careful.
Contrast this with BG2 and the De'Arnise Keep vs the Underdark Beholder Lair. The Keep is generally considered quite safe to do as the first thing once you're out of Johnny's Basemant, whereas the Beholder Lair is suicide unless you know what you're doing or you're using certain cheese items of beholder-nerfing. And yet both areas are level-scaled. It's just that one area is more aggressively scaled (with stronger enemies) than the other. BGS could learn a lot from that.
What level-scaling should've included in TES5 is a significant difference in the aggressiveness of level scaling (meaning "easy" areas and "hard" areas) and of course scaling through numbers. As overpowered as my Breton is, and regardless of how effortlessly she kills any opponent one on one, it remains rather unsafe to have 5+ Falmer crowding her and stabbing away. Alone they're walking corpses but in a group, they're still dangerous. Somewhat, at least.
At this stage it is utterly trivial to run through dungeons and never encountering more than 3 enemies at any given time. 3 enemies means they'll all be dead before my "slow time" shout runs out. It also means they'll probably all fall down with a shout of unrelenting force. In other words, 3 enemies are really not much of a hazard because they'll never actually get a chance to attack me in a group.
What if there were 10 enemies? 15 enemies? And not just pissy skeletons that die if you give them a mean stare, but actual, serious enemies that are all gunning for you? The Legion / Stormcloaks fort battles had some of this but there wasn't much tactics involved and you had a largely infinite backup force with you. Imagine a dragon priest throne room with an actual honor guard of 8 draugr deathlords? At earlier levels they'd just be regular draugr or restless draugr (and some of them might not come alive at all), but late game they're deathlords. Imagine if the dragon priest had a couple of lesser priests as well, who could cast a variety of summoning spells? I don't know about you but to me such an encounter would actually be fun.
Imagine if dragons stopped attacking settlements (where the dovahkiin has plenty of backup and fodder meatshields) and instead ganged up on the dovahkiin when he's isolated in the middle of nowhere, making it one dragon-born vs two or three or four ancient dragons? Sure, that much fire would murder my framerate, but who wouldn't be a little worried about turning their back on three dragons while dealing with the fourth? I know I would.
And the best thing about using numbers more actively in level-scaling is that it doesn't require all bandits to have glass equipment. it doesn't require 50% of deathlords to have ebony stuff. It doesn't require the extinction of all the normal low-level critters. A horde of very pathetic regular draugr is still an effective fodder meatshield that prevents me from getting to the draugr spellcaster, who is bombarding me with ice spells.
A horde of small spiders would still prevent me from getting to the giant spider, which would admittedly be more of a problem if giant spiders had a giant ranged attack they'd reliably use. Webbing, poison, acid, but just something that would force me to not just find a doorway and laugh at them. And who wouldn't want to fight 50 small spiders, 20 medium spiders, and one towering giant spider with a massive ranged attack and a sickly poisonous melee attack? Then again, I'm an arachnophobe and a general hater of spiders so maybe I'm just biased that way.