Limit is a great game. You learn the fundamentals, like TP said, but also important things like discipline, patience, and bankroll management. NL is fun because you can "double up," but don't forget how you can also stack off ... when someone gets lucky.
Limit is about capitalizing on the errors of your opponents, mathematically making correct plays, and grinding out those precious BB's.
No limit (at the low levels) is also about capitalizing on opponent mistakes, but you're really concerned w/ doubling up. Yes, there is math involved, including implied odds and drawing odds, but there's more "feel" and "instinct" invovled.
From my perspective, I think it's hard for the beginner to appreciate these subtle nuances, let alone be able to instinctually "feel out" or "read" a player online. Then again, maybe you can.
One last thing: as w/ the limit games offered online... I think they can be a HUGE cash cow if you have a sound pre-flop strategy, you stick to levels your BR can handle, and you learn the math. Now all you have to do is start logging hands.
It goes back to the earlier question of, "Why do you play poker?"
GL.