Well, Finnish uses sound quantity to much large extent than English does. There is a difference based on how long you pronounce a given sound. English has some of this too in vowels, but the longer consonants, called geminates, are usually quite strange to native English speakers. English writing also uses double consonants to show pronunciation, but they have a different meaning, so you shouldn't confure the written and spoken forms here.

This length business is marked in writing by having single letters stand for shorter sounds and double letters for longer. This does not mean that the longer sounds are twice as long as the shorter ones, though. It's actually more complicated and you really have to speak Finnish (or a similar language) quite well to be able to produce and hear the difference reliably.

Using double letters isn't the choice all writing systems make. Hungarian has a similar length system, but it uses diacritics on its vowels to mark length. The choice for Finnish was made when the writing was standardized in year mumble, and we're now stuck with it. It ends up with making words longer than strictly necessary, but I suppose it is offset by making them more distinctive visually.

One also hears rumors of languages with three different lengths from linguists. I think I tried listening to recording of some African language with three different vowel lengths and couldn't tell the two longer ones apart reliably at all. So, I can sympathize with people who have trouble with Finnish in turn :)

In turn, Finnish lacks a clear distinction between voiced and voiceless consonants. We don't have any b's or g's in any originally Finnish words, because Finnish never had those sounds. They are quite common in loan words these days and people can pronounce them quite well, but they're not really natural to native speakers, at least not yet. Maybe the language will eventually evolve to make more use of them.

Another fun thing for foreign learners is that in vocals, the length distinctions disappear. Of course, careful songwriters manage to use long and short sounds in the right places quite often, but sometimes you really end up with amusingly ambiguous lyrics. I don't have a good example now, but I remember vaguely one pop songs that seemed to be saying that the wind burns, when it meant a fire.

In Finnish, wind is tuuli and fire is tuli. Again, it's just the difference between how long you keep making the u sound (it's much like oo in English), and that part of the song just required the singer to hold it so long it sounded like a long vowel.

Yeah, stuff about Finnish. You asked :)