This thread was made last year, but erm, yeah...Tennis isn't that difficult to understand. It really isn't. As an avid football fan, I didn't pay much attention to Tennis, unless Wimbledon shows live on the BBC. And I just learnt the rules from there, I have pretty much learnt every rule of tennis just by watching it on TV. Football is very easy to understand, the offside rule is probably the hardest rule you can get with football, and even that isn't very difficult to understand. Someone mentioned Chess, I don't really consider that a sport, but again, Chess is also not very complicated, once you get the hang of it, and you keep playing it (like me), you just get used to it.
I don't watch much Cricket, but I know the basic rules of how Cricket is played. It's a very, very, very boring sport though.
With Rugby, I don't understand when it's a foul or when it's not a foul, other than that, I pretty much get it.