* Invasion of privacy - that's a big one, yes.

* Obnoxious and gratuitous animations - yep, that too.

* Pop-up/floating/video ads - static or near-static ads are fine. However, if it covers up the content, or makes a noise, it can go die in a fire.

* Badly-written scripts, or pages where there's half-a-dozen+ streaming video ads, can bring my creaky old laptop to its knees.

* There's a possibility (albeit small) of security holes in the browser's scripting engine.

* Personally, I consider graceful degradation a metric of the quality of a website - if it craps out without good reason, that reflects poorly on its owner, and is going to make me suspicious of the quality of their product.

I'm sure there's more, but that's all that spring to mind for me at the moment.