Indeed, usability is an issue that often stirs opinions and sets controversy - partly because heuristics is innately open to subjectivity and perhaps partly because many people are still not really aware of what usability is. I have gone through looong lists of dos and donts for usable web-sites, some of them darn good at describing common mistakes of us developers and IT professionals. What I would usually find in a top-10-mistakes countdown, however, is for me only half of the usability concern. Those are always specific, addressable issues like broken links, search-engine friendliness, reasonable use of graphics and color, concise web-style writing, fonts use, and page layout - all elements of what I call the "structural" usability concern. Long uncovered by usability experts, such heuristics serve predictably well in improving the user experience of any web-site. I believe, however, that their effect has an upper limit, set by factors beyond the immediate area of "structural" usability. At least for me, simply making a site search-engine friendly, with fluid layout and user-controlled font-size will not make it the most usable site, because usability is defined differently for each site - changing with site audiences and site purposes. With all of its compliance to usability standards, I doubt the layout of Useit.com would serve our visitors better than our current version. What I believe is really important in terms of usability, is to weave usability not just in the structural elements of a site, but in its conceptual design as well. What I call conceptual usability involves a genuine concern for users incorporated in every decision for the information architecture, layout, and most importantly - feel of the web-site. This concern needs, of course, to be backed up with both a solid understanding of one's users and a solid experience with web technologies and standards.
My point is that a list of usability todos can only improve to a certain level a web-site that has not been conceived with usability in mind. This said, for me one of the biggest usability don'ts is to content oneself with simply observing common usability standards - those will do little good if the web-site as a whole is not structured and laid out with concern for the users. I guess this is a part of the challenge of our profession, to know both people and technology, so we can create a meeting place that adds value.