What makes a User Interface good?
To me, good ui comes down to one deciding factor - Ease of use.
Before the ui is designed the client and agency generally compile and agree to a site architecture. although this stage comes before the actual ui design , it can be a key factor in how the ui turns out. If the client pushes for a site architecture that has upwards of 10 - 15 navigation items (i.e. Home, About, Philosophy, Products, Location, Background, Contact, etc, etc, etc) the chances of the site having a good UI are greatly reduced. On the other hand, a site architecture that consists of around 5 main navigation items with sub sections has a much better chance of success.
The rule of less is more comes into play here. A site that has 12 buttons accross the top for its navigation not only looks bad, it is also confusing to use. Additionally, if the companies business model is selling sprockets - the best way to insure that the sprockets are seen is to place the sprockets button next to as few other buttons as possible. This will increase its chances of being pressed and eliminate the chances of a user getting distracted by other navigational items. To do this its just a question of organization. For example, the items: management, background, philosphy, & careers could be combined into one item called About The Company then further divided into subsections.
Of course this is just my opinion, being more of a minimalist I tend to take things that are complicated and reduce them into their more basic form.