I always thought that they made Blood-Bending seem far weaker than it should be. They depict it as basically a puppetmaster controlling the targets movements. Yes a Blood-Bender could/should be able to do that, but the most significant way I envision a Blood-Bender using their power is to drain the very blood from their target, killing them in the process, and then using their blood as they would use water.
And Blood-Bending shouldn't be that rare/hard, the human body is 80% water. Why can't people just manipulate/drain that? I guess that would make Water-Benders too overpowered, since they'd always have access to their bending element (but so do Fire-Benders), but theirs would be lethal/unblockable..
Personally I've always wanted to be a Water-Bender, since I love water and ice, and it is the most fun/useful of Bending types in my opinion. And if Blood-Bending worked the way I see it working, I'd be unstoppable... <_<