A big pain point for clientside development is testing.
Setting up a good testing environment is really difficult and there isn't any good information on how to actually set one it.

I've figured it out through trial and error over the years, but very few developers actually test their stuff properly.

IE6 (XP), IE7 (XP), IE8 (XP), IE8 (Win 7), IE9 latest preview (Win 7), Firefox 1.0–latest beta, Safari 2.0–5.0, WebKit nightly, Chrome, Opera, iPhone, iPad, Windows Phone 7, Palm/HP webOS, etc… etc…