While I'm at it, I guess I can post about other things that folks find unpleasant.

PICKPOCKETING: Definitely a potential issue at some tourist spots. Old Town Square, especially at the astronomical clock. Metro/tram in the center. Charles Bridge. Keep an eye on your possessions. Keep your bag closed. Don't set anything down to take photos. Street scams, like the kind you might run into in Barcelona, don't seem to be that common. Actual violent crime is next to nil. I've been alone in dark parts of PRague at 3am and felt totally safe in a way that never happens in a large city in the U.S.

OVERCHARGING/RESTAURANT SCAMS: This is beyond the inflated pricing that goes with being in the tourist center. (But about that...you can pay 3 or 4x more for a beer and about everything else around Old Town Square and the Bridge area than you would even a couple street away). You see a sign that says "beer, 40 crowns," walk in and order a beer. You get a half-liter and a small bowl of pretzels. Turns out that the 40 crown beer is .3L, but your .5L beer is 100 crowns and the pretzels are another 20. Ask to see a menu. Don't eat anything that you didn't order without finding out how much it costs.

DRUGS: The Czech Republic has decriminalized small amounts of drugs...to my knowledge, this is virtually all drugs, even hard ones. It is very common for people (even reputable people) to smoke marijuana in parks, beer gardens, and sometimes just walking down the street. Much rarer: I've seen people shooting up in the last year or two. Like, sitting on park benches on main streets. Maybe three or four times, so unlikely you'll see it, but don't be shocked.

PUBLIC URINATION: Happens. There are more free public toilets now than a few years ago, but the combination of a people that like to drink beer with pay toilets = people (well, almost always male) urinating in the bush, on the sides of buildings, etc.

DISPASSIONATE SERVICE: Those who are staying at nice hotels, eat at nice restaurants, and generally stay in the tourist zone won't encounter much. Americans, in particular, though always comment on the attitude of shop clerks, waiters, etc. if you hang out in 'locals' places long enough. Attempting to buy 80 crowns worth of something with a 1000Kc bill (or even 500 sometimes) can lead to a mild temper tantrum or a cold stare. There's also this passive-aggressive streak that can run through the culture.

SMOKING: To go along with the pollution warning above, I should say that most authentic Czech pubs are smokey, poorly ventilated affairs. Smoking is a matter of course on streets, in parks, at beer gardens, basically anywhere outside.