It's convenient to use feet because planes are stacked in 500' increments and not 152.4m increments. VFR (visual flight rules) flights are usually on the 500's (eg. 3500', 4500', etc. depending on heading) whereas IFR (instrument flight rules) flights are on the 000's (4000', 5000', etc.).
Knots are convenient because 1 knot is equal to 1 minute of 1 degree of arc on a great circle. If you're flying anywhere far away this ends up being important as a great circle is the shortest route between any two places.
Oddly enough, the metric system is useful with temperature because the standard lapse rate is 2 degrees per 1000' of altitude. So if you had to climb from 6000' to 8000' on an IFR flight plan, you would usually drop 4 degrees centigrade, which might be significant if it was raining out and it dropped below freezing. Having water on your wings and climbing up to an altitude where it's freezing is going to make you have a really bad day.