The ever-increasing intrusiveness of advertising: popovers, always-visible ad footers, slide-outs, slide-overs, flyovers, singing dancing twirling until "OMG just make it stop!" I resisted AdBlockPlus for a long time, but now I've embraced it completely, and I block everything all over the place using the broadest possible filter parameters. I'm sure I'm now missing content that I'd actually want to see, but I consider it a small trade-off for getting rid of the annoying crap.
Also, forums in which everyone always quotes in full every message that they're replying to, including multiple nested replies and huge layout-breaking images. Often you have to scroll past 5 screens of quoted material to get to a 2-word reply.