Academic excellence
This is even more important a consideration than it was at undergraduate level. Although there is a correlation between excellence as an undergraduate institution and excellence as a postgraduate institution, it is not perfect. The rankings that most departments care about the most (in part as it determines a large amount of their funding) are the Research Assessment Exercise rankings (RAE). The rankings vary from 1 to 5* (where 5* is ‘world class').
The RAE tables are made infrequently. Here are the results of the most recent RAE which was completed in the December 2008 RAE. The 2001 RAE results are also still available. The RAE results are a good place to start but departments are likely to change.
Academic specialisms
These are also important. If you intend to do a PhD after your Master's, you might already have an idea which general area you would like to specialise in. In this case, it is worth asking your lecturers which departments are particularly strong in a certain field. You might even tentatively identify some academics to approach as supervisors.
Fees and Financial support
This varies considerably, as mentioned above, and you should think about it carefully. Of course, courses which charge higher fees might be better (price might be a signal of quality) so don't just go for the cheapest option without thinking about it. More information on funding is available on the funding page.