Since no one's addressed design, I'll add my 2¢.
The sites that I go back to on a regular basis have three things in common (as related to design):
Original: The site must be unique in some way. Granted, it shouldn't be so unique as to make it difficult to navigate, the advice about putting navigation at the top left is good, it should always be where the user expects it to be, but overall, the design should not look like it was ripped off of anyone else's site.
Clean: The design needs to contain the right amount of white space, needs to be uncluttered, and above all, needs to direct my eye to the important information on the page. If I visit a page to read an article, I need to be able to read that article without first sifting through all the other visual junk on the page. If I can't quickly find the information on the page that I'm expecting to see, I hit back or close within just a few seconds usually.
Consistent: Don't change designs for different pages of the site, obviously, but also make sure you're using the same design elements all over the page. Don't mix reflections and drop shadows, don't make one part of your page blue and white but suddenly make another element on the page green for no good reason. Keep color schemes and layouts consistent both throughout the site and on each individual page.
I can show you a site that I've made that I've strived for those three things, although recently the last one has been screwed with a bit (due to the company wanting particular things up but not wanting me to change the entire design). It's not a great site by any means, but I think it's a good site overall.
http://www.connectingpt.com/
And one that I don't think is as good (also mine):
http://www.japancpi.com/
See if you can note some of the differences. Feel free to point out any mistakes, too, I'm not going to be offended (though I may not have the option to change things).
jW