I was in London a few weeks ago and I thought I'd post my best and worst.
Best of London in no particular order:
1 - The Beefeater tour at the Tower of London - tickets to the tower are expensive but the tour is included. It's crowded but worth it.
2 - St. Paul's Cathederal - So big, so empty. Great place to get away from the crowds. The climb to the dome nearly did me in, but the views were incredible.
3 - Westminster Abbey - beautiful, don't miss it.
4 - the Tube - possibly the best public transport system in the world. Easy to navigate and reasonably priced (with an Oyster Card). We used it constantly. The circle line was a little slower, and a few stations were crowded, but the trains were fast and reliable. Note that Black Cabs are also as nice as everyone says they are.
Worst of London -
The changing of the guard at Buckingham Palace.
We arrived three hours early to get a good spot and still ended up being elbowed out of the way by a couple of extremely rude Italians and a marginally more pleasant French school group. Most of the action takes place behind the gates and is difficult to see. There had to be 6000 people milling around. If you've seen it on TV, I'd suggest that you skip it.