Altura panniers, tough as and fully waterproof with the cover on.
Yep I'd second that. I have the 56L ones into which you can get an amazing amount of stuff.
Then I'd say mudguards- I remember an old bike I once had without them and I was forever chucking clothes away that were ruined by oil and crap off the road.
My dynamo lights are good and stay on when at a standstill for some time. They also come on automatically when they sense darkness- a bit daft I know but I quite like it.
Finally I wouldn't be without my kickstand. The last one broke back in May and I was without one for a couple of weeks. I hated propping the bike against trees/ fences etc - scratched the paintwork to buggery. The stand also allows for easy lubing and gear fettling as you can lift the back wheel easily and turn the pedals clockwise.