The fast-paced stepchild of the sport you may only be partially familiar with is almost football. You will love it.
My rugby fandom was a product of circumstance more than anything else. Thanks to an expansive sports package on my parent's cable and mild insomnia, Canterbury vs Auckland at 12:30 AM on some derivative Fox Sports channel became appointment viewing. The Americans were never any good -- they've mostly gotten the crap kicked out of them -- but that's all started to change and the team was highly competitive this year in the build-up to the Olympics. I can tell you this as a football fan, Rugby 7s might be the most exciting sport played on grass.