There are a lot of great places to go in Europe, but few cities have enough to keep one occupied for ... ever, like London. It's one place I've been multiple times now, and in each visit I find something new.
What makes London such a fantastic place to visit is that it has everything, all of it is fairly compact, and the city is easy to navigate. London's Underground, for example, is famous for a great reason -- you literally can get anywhere in the city fairly quickly. There is not a spot in London more than three hundred yards away from an Underground terminal. Compare that with the equally important but far less robust Washington Metro.
Part of the challenge of deciding on an itinerary for London is figuring out what you want to see or do. You never have enough time to do it all, and I personally believe the time is better served by doing a few things in depth rather than try to hit a little bit of everything. The first time I went to London, I had a full week to explore, and was able to devote the time to several attractions that needed it. As an example, the British Museum cannot be visited in less than a day unless you rush through it. It's too big, and too diverse -- you'd miss a lot if you tried to condense it into a half-day. The Tower of London and the Tower Bridge are a full day also if you take your time to learn everything about it. Too many people try to rush through it and wind up missing the full experience. Buckingham Palace, when it is opened, must also be visited cautiously because you will invariably want to see the changing of the guard, but that will chew up a lot more time than you think.
I divided this travelogue into readily digestible subchapters, although during a future trip to London I will probably take the City Tour chapter and divided it further. The Thames River and London Tower chapters clearly represent day trips to themselves. Click one of the highlighted areas on the map to access these chapters. (Note: Currently the Buckingham and Thames North Bank are combined. They will be split out in future.)
The map comes from the Microsoft Expedia.