There are a lot of things I hate... this will be a long long list... :)
1) Popup ads. I don't want any ads throwed on my face. My browser windows belong to me and no one should hijack them. Especially the popups advertising online casinos make me scream because casinos couldn't interest me less. Fortunately I have popups disabled when I surf the web.
2) Websites that tell me to use a certain screen resolution, font size, or otherwise change my settings. My screen and system belong to me and no one should tell me what to do with them. Why should I change my settings in order to view a website that is obviously made by a lazy idiot who doesn't want any visitors anyway?
3) Websites that tell me I'm not using IE and require me to download it before I can view the website's content. I can't even download IE because I'm not using Windows, and even if I was using Windows, I wouldn't want IE. My software belongs to me and no one should tell me what to use.
4) Splash pages or flash intros. Hate it, hate it, hate it.
5) Websites with flash in general. I'm on a 56k connection so they usually take too long to load.
7) Scrolling things in general. Blinking, moving, jumping images and the <marquee> tag also make me scream.
8) The disable right-click script. My right-click menu belongs to me and no one should hijack it. People use the right-click for many other things than stealing code or pictures. If someone really wants to steal code or pics, it's still very easy to do itdespite the disable right-click script. People who use such a script are paranoid idiots.
9) HTML or XHTML code that isn't standards compliant. The usage of nonstandard tags is one reason why people require their visitors to use a specific browser in order to view their website. This is a clever trick from Netscape and Microsoft.
10) Netscape 4.x and older. I don't understand why anyone would use NS 4.x these days. This is the 21st century! NS 4.x doesn't render tables correctly and has problems with CSS, thus making it harder to make standards compliant websites. Why make websites that aren't standards compliant just to make them readable in an old, obsolete and broken browser?
11) WYSIWYG editors. They produce very messy code and overuse the evil <font> tag and make huge pages that load very slowly. Some WYSIWYG editors also link the pictures to the user's hard disk, causing a lot of confusion when the user uploads his/her web pages and wonder why the images don't show up.
I know I sound like someone who hates the web, but naah... I still like it. :)