It's interesting looking at the different densities of the systems. London is rarther in between, covering a fairly large area, yet being quite dence in the middle. You've then got systems like Paris, which is much more compact, with lines going nothing like as far out (although if the RER system was added that would change), yet being a mad tangle in the centre. Tokyo is similar, but extends a bit further out. Another thing is that whilst most systems are fairly round, both in terms of the outer boundries, and also there inner (and other) circles, London is far more flattened, infact North-South it's pretty much average size, wheras East-West it's around twice the size of most other systems. Then there are things like San Fransisco and Los Angeles, which cover very large areas, but are quite simple. Oviously, those two are a lot more modern than London and Paris (not sure about Tokyo). Los Angeles actually looks like an upside down stikman, even with a little head, which I presume is a turning loop, although I won't imagine what the branch coming off near the top of one of his legs is. The modern systems tend to be a lot straighter than the old ones, due to not needing to follow roads.
A few other systems it misses are Newcastle and Glasgow, and also Lille in France.