Throw in lots of animated GIFs
Just because they look so cool/cute. Everybody must like them.

No, everybody doesn't like them. As a matter of fact, some people are downright infuriated by the things.

Having animations running constantly can be distracting when too many are used. They can also get reeaallyy annoying, reeaallyy fast.

Don't get us wrong, it isn't as if you can't have any animations or anything like that on your page. One or two lightly sprinkled here or there around the site can be just fine. Unless you absolutely need more, for some clearly defined reason, there should be no more than one animated object in your visitor's field of view at any time. Ever.

One thing you should avoid AT ALL COSTS is: having tiled animated background images. Those things can drive a body nuts faster than just about anything.

In case you're wondering:

large, flashing,

obnoxious ads

fall into this category, too. Don't do it.