One thing that really peeves the hell out of me is waiting forever for a page to load when I shouldn't have to. Too many sites are full of loads of graphics, or large graphics, that take forever to download, and then you find out that it wasn't worth waiting for, or that none of them were essential.
A good “bad” example used to be the Yellow Pages (the Australian business phone directory, in case there's another). It took an age to fetch, because they'd used graphics all over the place. It was actually quicker to drag out the printed book and search through it by hand to find a business than use the website.
And then there's sites that insist on using Java, JavaScript, or some other executable code, when it's not really necessary. I, for one, aren't going to be continually patching up my web browser with updates just to use your page, when it's not even doing anything more than could be done with just HTML, nor am I really trusting enough to run an unknown executable code on my computer either. Even disregarding maliciously programmed web sites, there's an awful lot of extremely badly programmed ones; and I'm damned if I care about running someone's buggy codes, just because they think it looks pretty.
Then, if you're in the position that you must use a text based browser to surf the internet (blind people, for instance, so their speech synthesiser can read the contents to them), then you're stuck, as less and less sites are usuable without the latest all singing and dancing graphical based web browser.
But the stupidiest situation of the lot is; commercial sites that can only be viewed by one series (or one particular release even) of a web browser. Just what is the point of making an advert if only a small fraction of your potential customers can actually see it? Especially in the situation where a computer firm is trying to sell their new whiz bang computer to people who have an old computer (to get them to upgrade), but they can't even view the page on their old computer.
Since writing this page, I've also written another page, about morons who make bad web pages, which is much more critical. It's what I use to reply back to morons who justify their crap pages with dumb excuses.