I really like the ease of use and overall look of the site, the forum in particular is very user-friendly IMO. The administrators that I have dealt with are all incredibly friendly and I like that they are always open to suggestions regarding possible improvements to the site.
On a related note, I think it's brilliant that the site is constantly being updated and tweaked based on these suggestions. The Amazon and Argos bargain pages are immensely helpful and save a lot of time trying to find where something is cheapest. I like the fact that although the site is UK-owned and run, it tries to target a broader audience and has a global perspective.
I like the fact that Brickset tries to maintain a good relationship with TLG by respecting their wishes with regard to leaked photos and sets, and that as a result it seems to me to be a more credible site than others. The overall look (lack of advertising and not overly cluttered) of the site and tone of the articles back up this credible impression as well.
The information contained in the database is clear and concise and, as far as I can tell, accurate. The photographs are of excellent quality and I like the feature whereby you can enlarge the images by clicking on them.
I love the Brickset forum, I've made a lot of good friends through there. The dicussions are for the most part interesting and the level of moderation is just about right in my opinion. I think it is an excellent thing that the 16+ rule is in place as I believe the quality of the discussions would severely deteriorate if this were not there.
Overall, I probably haven't remembered to mention here everything I love about this site but suffice to say that I think it is excellent and I now never go elsewhere for Lego news. Keep up the good work!!