Is DNN perfect? No, and neither are any of the other implementaitons they speak of! I don't recall having any speed problems since DNN 3.x.
Is DNN bloated, in some areas perhaps -- but to do everything that people might want to do with it -- it has to be a little bit. I feel the team has addressed this somewhat by allowing you to easily uninstall even the out of the box modules so you are only loading what you need. I typically only install the HTML module initially. If I need Links or IFRAMEs, etc. I add those modules as I need them.
The W3C validator is a bit of a joke at times. It will fail and return multiple errors if it detects one tag it doesn't understand. ie. If you programtically set your header title, it will say it doesn't know what "id" is.
I think the MUCH more important test is how does a page load in all modern browsers. After all, your end audience should be humans using a web browser, not a machine. I test in Internet Explorer 6 *cringe*, Internet Explorer 7, Internet Explorer 8, Firefox, Safari, Chrome on the PC and I test on Firefox, Safari, and iPhone Simulator on the Mac. I also test using Lynx Viewer to see how the sites are seen using screen readers, etc. I look at my Google indexing reports to ensure that Google isn't stumbling over anything. If all appear okay...