... at how lazy American graduate students are. I returned to University after 7 years absence and that's been the most difficult adjustment. I've gotten used to those around me being willing to work 50 hour weeks every week. Graduate students complain if they have to work 20 hours. It's been great for me ... all my professors think I'm much brighter than I really am. It's easy to fool them since I just treat my school work like a real job and try to spend at least 8 hours each day working at it.
There is one skill that I'd recommend you try to pick up immediately, though: RESEARCH. You need to learn two things. First, you need to learn how to find papers about what you're interested in. And, scholar.google.com isn't sufficient (yet). Second, you need to learn to actually read and understand those papers. Sometimes it can take a month just to get through a single paper. But, learn how to push through and you'll reap dividends. Some graduate departments do a good job of teaching research skills. Others are lousy. Your experience could differ alot. But, to be good in graduate school requires alot more than just attendance. So, learn to do good research before you're paying to do so and you'll be happier.
As far as admissions goes, I think you should call the department you want to study in and ask the graduate coordinator. Every department is different. Some Universities really cater to returning professional students -- George Mason University in Virginia is that way. Other Universities have different philosophies.
In my field (mathematics) most admissions decisions are ultimately made based upon academic recommendations. I think that GPA and test scores are important, too. My wife says that when she worked in the admissions office they used GPA and test scores just to sort the applications into "bother to read recommendations" and "don't bother" piles. YMMV.
I can't speak to the differences between US and UK schools. I've got a number of friends who went to school in Canada and I don't think our systems differ much. Except Canadian schools are better funded so they're cheaper.
If you're trying to decide where to apply I'd recommend that you start by reading conference papers and published research. Try to find some research topics that are of particular interest to you. Then, look at where there are academics who are currently working in that field. You may discover that the preeminent researcher in your interest is at a really cheap school! How cool would that be! If you have no idea what you want to do with your life, then try to find a department that's large enough to support lots of research interests. That way, you can used your Masters degree as an opportunity to sample everything and see what you like best.
If you're returning to school because you just want another degree (and eventually a better job) then you should consider trying to make sure you stay very well rounded. On the other hand, if you're trying to become an academic you need to worry more about when you're going to start publishing. Even small teaching colleges are starting to look for applicants to have multiple published papers. It's by far best if you get a couple of things published before you finish your PhD. Otherwise, finding a job could be tough. So, if this is your goal you may want to try specializing soon enough that you can start working on papers. Again, YMMV.
Finally, I just wanted to say that both my wife and I returned to graduate school after many years away and we've both had a blast. I hope you have a similar experience! Good luck!
