Blue shells are fine. I usually develop a strong enough lead so it doesn't affect me much anyway. (Unless multiple get sent, which is annoying but fair, I suppose...) What I don't like is when people know when blue shell has been launched. A blue shell is supposed to hit the player in first place to give the others behind a chance to catch up, but when players literally reverse to avoid being hit, it completely defeats the point of it.
Also I can't stand thunder cloud. I hope it's only exclusive to Mario Kart Wii.