I must have done something to screw up textures. No matter the graphics settings (low / ultra), a lot of things have low textures. I didn't really notice it until recently, and now clothes / armor and items shown in the loading screen are blurry / low-res; I haven't looked at much else, but the game just seems to look like the quality was lowered to something below low. Any idea what the problem is?

Skyrim just comes with low rez textures compared to the high ones you see in mods.

I played on low with no real graphical mods, but now some textures look even worse.

That is normal. While many of the objects you may have added through mods may have increased the detail the texture models, many aspect still have lo-res images. Take flowers for example, even with the Skyrim HD textures I use, those still look terrible and they are two dimensional objects as well. Oblivion's flowers were nearly all three dimensional.

But the textures look worse than they used to, without even changing the graphics. The things looked current generation quality before, but now some things look like they belong in a game from 10 years ago (blurry, harder to identify edges). Can mods also decrease the detail of some things (even unintentionally)?

I feel as if the quality have sunk a bit too.
Not the reeolution, but my AA doesn't seem to work.
Everything is a bit blocky.

Maybe I misunderstood. Are you saying this wasn't an issue, but now it is? Because for me, I still see some of those blocky looking terrible lo-res images in Skyrim. Removing the texture pack I use now brings all of them back. The design of the game was to improve the aesthetics and realism of the overall looks and effects (like waterfalls for example), outside of really zooming in on objects up close; they did a good job on this. But the vanilla game still uses those ugly lo-res images like those used in Oblivion when viewed closely. There's just no way they could have made all of these environmental effects work without using ultra lo-res images and then expect them to work on the consoles. The console version was (sadly) ported over to PC.

Are you, per chance, using the vanilla settings and also using your graphics card settings at the same time?

OP, check your graphics card settings. You may have accidentally lowered mip-map quality. (I had this problem, too. )

In CCC, the mipmap slider is at the far left, which is max "performance". Is that bad?

And low-res things wasn't a problem before.

The more you lean that option toward "Performance", the more smoothly your GPU will run Skyrim. Problem is, you get this performance boost by lowering the quality of the textures. Set the slider as much toward "Quality" as you can. If you have even a 5000 you should be able to push it to the far right.

I just put it to the far right, and it didn't seem to affect performance much. It was exactly the problem though. It looks much better now. Many thanks.