More than anything, it really depends on the area. The amount of stations allocated, the frequency range used, and the wattage/height varies from country to country.
Conventionally, the FM band in Europe goes from 87.5 to 108.0 in 100 kHz increments. Some countries, like Italy and Greece, also have stations in 50 kHz increments (e.g. 93.45). In some countries, some frequencies are unused (the band might start at 87.6 instead of 87.5 or end at 107.7 or 107.9 instead of 108.0).
In the UK and a few other countries, public broadcasters like the BBC have a big chunk of the spectrum to themselves...much like non-commercial stations have exclusive use of 88-92 mHz in the United States. In other countries, public broadcasters seem to be allocated frequencies more at random, all across the dial. This is the case especially in Southern Europe. RAI, in Italy, for instance, does not have any part of the dial belonging exclusively to it.
As far as wattage goes...in the UK, Ireland and most Northern European countries, state broadcasters generally pump out very powerful signals (10-100 kw) along with many lower-powered repeaters to fill in the gaps, while commercial stations are limited to 2-3 kw, or even just a few hundred watts. However, in countries like France, Spain, Italy, Greece, etc., this does not seem to be the case, as many commercial stations put out as much, if not more wattage than state/public stations. So stations in Europe do not universally run much less power, but that is the case in many countries, especially in the north.
A perfect example is in Athens, Greece, where the dial is crammed with stations. Legally, most stations operate 300 kHz apart. So you'll have, for instance, stations on 87.7, 88.0, 88.3, 88.6, etc. They all put out rather high power, probably anywhere from 5-20 kw, and most of them broadcast from the same location. Reception of these stations really is not a problem, though sometimes certain stations may "bend the rules" and up their power a bit, stepping on their neighbors. Usually though this isn't a problem. When I traveled there, I had no problem receiving even stations that were 200 kHz apart in some cases. I know the radio band in Italy and Spain and even France can also be crammed like this in many areas, especially major cities. Ultimately, it depends on whether or not the stations are all broadcasting from the same location, at similar wattage and with the proper equipment.
Final interesting note...there may be relatively few FM stations in the UK, but major cities, and especially London, have had most of those empty frequencies filled up by pirates. I believe London alone has around 80 or so at any given time.
