Why is Thor's chariot pulled by goats?
Thor's "ride" fits with his role as god of the common people. While most of the gods ride horses, Thor drives a wagon, or walks. (He walks across Bifrost, the rainbow bridge, according to the Prose Edda.)
As for the goats, I'm just guessing here, but there's a verse in the poem Havamal, which is supposed to be the widsom of Odin himself:
One's own house is best, though small it may be;
each man is master at
home; though he have but two goats and a bark-thatched hut
'tis better than craving a boon.
So a goat would be the least that a subsistence farmer could get by with - but Thor's goats are special, because he can eat them and use them for draft animals time and again. Also, anyone who can yoke a goat and make it pull a wagon is clearly possessed of supernatural powers.
Hmm... I do not disagree with the gist of this answer, but I can think of only four gods for which we know a means of transport except walking: Thor drawn by his goats, Odin on Sleipnir, Frey on his boar or his ship, and Freya drawn by her cats or (presumably) using her falcon shape.
Heimdall and Freyr have horses, at least in the Prose Edda, but I take your point. However, Thor walks across Bifrost, and the whole point of the poem Harbardsljod is that Odin (disguised) can refuse to ferry Thor across a ford too deep to wade. But it's true that we don't know how most of them get around.
I just thought of something - goats aren't common in Norse myth, but the goat Heidrun provides mead (instead of milk) for the inhabitants of Valhalla, enough for them all to drink their fill each day. So I don't know *why* goats, but there seems to be a theme here.
You might find this essay on why there's goats but no sheep in the myths interesting. I find it a little too speculative when there's only two uses of one and none for the other animal (and sheep as draft animals for Thor seems very odd), but it's interesting anyway.
I'm glad I used the term "weird" per it's association with portentousness re: "anyone who can yoke a goat and make it pull a wagon is clearly possessed of supernatural powers."
@andejons: In Wagner's *Ring* (specifically *Die Walküre* 2.1, stage direction by the poet/composer) Fricka arrives in a chariot drawn by two rams.