Very, in my opinion the easiest CMS to skin for. Best way I find is to design your layout in Photoshop (good practise to use 960 Grid for this), and transpose these dimensions/images/slices into HTML/CSS. You can then split up the html/css into the relevant header, page, post etc Wordpress files. I would also recommend developing from a base Wordpress framework theme, such as Hybrid or Thematic.
Totally agree... unfortunately there are some plugins that I've used to try and save time which turn out to produce invalid code. Downloads manager for one of them (by the way, don't bother using this! I wish I hadn't...). Out of interest what page was that on? Before I utilised a couple of different plugins the original template is valid XHTML 1.0 Strict.
EDIT: Looks like the front page, I'll get this sorted methinks! Bear in mind that it is still very much W.I.P. but seeing as it was such an improvement over our old E107 site it went live