This is a way of asking for something. Depending on the situation, it can seem either polite or angry. Here are some situations in which it seems polite:
I'd appreciate any change that you can spare.
I'd really appreciate it if you could pick Manny up from daycare and watch him for a while this afternoon.
Here are some situations in which it seems angry:
I'd appreciate it if you'd tell me before doing something like that.
I'd appreciate a little more effort in practice from you.