I'd say the common american street fashion is...
Conformist & Simple
In Japan, the street fashion is in a "whatever you think is interesting & brings attention, where it" kind of feeling so there's no trends to really follow except for your own. No need to explain why Japan's fashion isn't simple. :P
Here in America, the fashion changes season by season so every store sells similar styles of clothing as one another. (except for Hot Topic... their style stays the same forever and ever. :] ) American street fashion is simple yet appealing.