The reason there is an armour for American Footballers is that you can basically tackle anyone on the opponent team. In rugby, you can only tackle the guy holding the ball.
Why don't rugby players wear armor? Well, there wasn't any armor during the medieval ages, so tradition just kept it that way. No armor. Although, many are sporting head-gear but not actually helmets, it's more like a pad on the back of your head.
Another thing I find fun with rugby is the faster pace of the game. In rugby, if you drop the ball, pick the ball and play on. Drop it three times, the other team gets the ball. In American football, it takes near to 50 years before you get the game going again (I was sarcastic, it takes around 2-3 minutes).
But there is one thing I don't like with rugby. I know who the guy is going to pass to, but I'm unable to touch him until he gets the ball. Had it been American football, I'd dropkicked the dude before he even sees the ball coming.
Another thing is that, in rugby it's so darn hard to advance forward unlike in American football. You can't pass to people who are infront of you so that means, if you are infront of your team, you're more or less useless. Thus, the teams moves in lines, with the guy infront moving to the back, and the guy at the back with the ball moving to the front and passing the ball back before he gets tackled. Strange rules, aye? In American football. See the big dude infront? Give him the ball!
-Taison out