In the first few lessons it was definitely roundabouts. I'd either miss openings or go too early. The later ones it was mirrors; after my first test failed I moved house and with it, county. As a result I was put on a completely new set of test routes and roads and there were lots of narrow roads and bridges - what didn't help was the 40mph speed limit that accompanied some of these narrow bendy roads. Overall though, planning was probably the hardest part of learning to drive to me (and also failed my first exam). Control becomes a joke once you're used to the car and have done enough turns to basically autopilot your control; which in turn allowed me to focus on my observation and planning.
Early lesson stalling was a big one too; I didn't follow the golden rule - lifting your clutch past the point your car is already moving isn't going to make you move off any quicker; it's going to do the opposite.