1. Migration, mostly; migrants come often in the springtime, although occasionally in summer and autumn; this can depend on your area's climate and location, I believe, as distant, barren, frigid/desert lands are understandably less likely to attract immigrants. (edit: married dwarves can also give birth to children, which will of course eventually grow into working dwarves, but this happens only once in a while, takes a long time, and is definitely not your primary method of getting new dwarves).
2. Just disgusts your dwarves, makes them a bit unhappy. It fades away naturally, but to prevent it from HAPPENING, make a refuse pile OUTSIDE your fortress; miasma is pretty much just noxious fumes from decaying corpses, so moving them outside to a trash heap gets rid of the problem. (edit: if you don't want your dwarves wandering around outside, you can also put your refuse pile indoors but behind closed doors so the fumes don't escape.)
3. Goblins and kobolds will start attacking a few years in, 4 or 5-ish; humans and elves will only attack if you mess with their traders. Random creatures can also come out of the river, chasm, and magma.
(edit again: damn, looks like my drawn-out reply got me beaten to the punch, ah well. >_> )
[ July 01, 2007: Message edited by: The Moonlit Knight ]
[ July 01, 2007: Message edited by: The Moonlit Knight ]