As has been flavour-of-the-week on UX.StackExchange this week, I think a Heuristic Evaluation is what you're looking for. It's all very well us telling you what is good and bad about the website but nothing beats doing some analysis yourself. That'll give you a better understanding of what makes a website provide a good user experience.
Heuristic evaluation is the most popular of the usability inspection
methods. Heuristic evaluation is done as a systematic inspection of a
user interface design for usability. The goal of heuristic evaluation
is to find the usability problems in the design so that they can be
attended to as part of an iterative design process. Heuristic
evaluation involves having a small set of evaluators examine the
interface and judge its compliance with recognized usability
principles (the "heuristics").
Rather than determining what is wrong with the site, you can assess the site for what it does well (even suggest improvements, there is no 'perfect' website).
A couple of questions from this board this week were discussing this with some great useful feedback:
Heuristic approaches for users' evaluation of UX
(As my answer is shamelessly stealing from these two questions / answers if you feel the need to upvote my answer please make sure you go and upvote the useful answers in those threads too)