You know what I hate on webpages? (Yes, I know there are a load of things I hate. Sue me. :p)
Some of these are more personal annoyances than anything... but eh, what the hell:

i. Horizontal layouts. GodDAMNit, they're annoying. I despise horizontal scrolling. Especially if they're blog layouts, so I have to scroll to the right AND up and down to read the entries.

ii. A certain shade of pink/purple/whatever. Whatever the goddamn color is called, I hate hate hate hate HATE it. Now if it was used tastefully, I wouldn't mind as much. But when an ENTIRE layout is done with that color... *winces* I just stumbled on a site like that, and my eyes hurt. x.x
More in general: COLOR COORDINATION, PEOPLE. Try to have a little respect for other people's eyes when choosing the color scheme for your layout. Dark green on hot pink is bad bad bad. So is red on highlighter green. So is very light blue on white. So is anything that is difficult to read.

iii. Design over content. Yes, yes I know, I'm semi-guilty of this myself, I make new layouts for my sites and never update the content... *sheepish grin* But hey, I'm lazy. And I don't have much spare time, and layouts are quicker to do for me than content updates. Plus, I know I should update my sites' content more. But I see people who say something like, "I was thinking of opening a new site, but I decided to make a new layout instead, because hey, who cares about content?" ><

iv. Related to the previous annoyance, but this is specific to blogs. Now, I - and I'm sure many others do the same - link from their blog the other blogs they like to read. I seriously can't understand people who link blogs because "they look good". If the person writing the blog has the personality and mental capacity of a vegetable, and all they can do is say "OMG LOLOLOL mwah I luv yah" and stuff like that, then the beauty of the layout is worthless. It's like meeting a gorgeous person and realizing that they are incredibly stupid.

v. When I open a webpage (keep in mind, I have a FAST connection) and I look at the status bar and see... "37 (or some other high number) objects remaining". And I minimize that window and go visit some other websites or chat with friends on IRC, and after five minutes I come back and it STILL hasn't loaded. Talk about image heavy. I don't care how beautiful a webpage is supposed to be, if after five minutes it hasn't loaded on my fast connection, I close the window and never come back. It's especially bad when it's a blog. You don't need 300K of images on a bloody blog. ><