I have been using Premiere extensively for a couple of years now, and would consider myself quite proficient at it.
There are, however, a few features that I've not yet learnt how to use. Namely the 3-point and 4-point editing functions and single track (as opposed to A-B timeline) editing. I don't know anything about these features at this stage. However, if they are significantly quicker than using the normal timeline editing method, it would be worth my while learning how to use them.