I have three 'problem' areas. 1) a high traffic, narrow, no shoulder, two-lane road, down hill, ending with a left turn onto a bridge. If there's no traffic, it is fine -- I can cruise down at speed. But normally there are cars backed up for the entire length, which means I either have to ride the right side -- but often the cars are pulled too far to the right for me to comfortably pass -- or ride the center line, in which case I need to ensure that I have space to get out of the way of oncoming traffic. The worst part, though, is the end -- the traffic coming from the cross-road has horrible line of sight, and I feel very vulnerable (not to mention folks here have mentioned being hit, or seeing other cyclists hit at this spot).

2) On the way home -- to avoid that same stretch riding uphill -- at a glorious 9-10 MPH, in my case -- I have a very steep (15-18% grade, as best I can tell) hill, which is presently covered with leaves. I had to walk up one day when the leaves were wet.

3) I have a towpath section which is rocky and not very bike friendly, even on my larger tire hybrid commuter. Someday I'm borrowing my brother's MTB just to see how that would be.