Czechs can appear very serious; they don't smile readily in public, tend to keep themselves very much to themselves on public transport, and are generally even more reserved than the British. Quite often waiters/waitresses can also seem a bit curt, but it's just a cultural thing - you soon get used to it; just don't take it personally, they're like that with everyone! Underneath it all they are. by and large, a warm, friendly, welcoming nationality and are nothing if not polite - even stiffly formal in some cases. Having said that, things have been loosening up in Prague (not so much so in other cities, and certainly not out in the sticks) over the last few years, so that nowadays cgeerful faces are becoming a little more common and service is becoming warmer and more open. I've spent so much time in the Czech Republic over the last 9 or 10 years that I don't notice the serious faces any more!
As it's your first time in Prague, I'd like to echo fairkatrina's warning about pickpockets; actually they're by no means the worst in Europe, but they are a real nuisance, and the last thing you want is to have to spend hours in a Czech police station (not the most cheerful of places) while you make your formal complaint (necessary if you want to claim on your travel insurance). Also, I'd advise against the taxis - they're overpriced and driven, on the whole, by lunatics. Make full use of the cheap and comprehensive public transport - the tram is the best way of getting about . DON'T under any circumstances change any money with anyone who approaches you in the street.
You'll have a wonderful time in my favourite European city - enjoy!