Bike difficult to pedal I added a new rear wheel to my bike, and set the chain and spun it by hand so it spins properly. However it is very very difficult to pedal, requiring a lot of force. Also when I pedal, I hear a light grinding sound that I can't identify. Does anyone know how I can fix this?
This could be a number of things. Any of the chainset components, hubs, brakes.

To narrow it down, 


Take the chain off the front chainring and pedal by hand, do you feel any resistance here? If so this could indicate a problem with the bottom bracket
Spin the wheel off the ground by hand in a forward direction. Does it spin freely and continue spinning when you let go? If not this could indicate brakes or hub bearings 
If neither of the above are the problem then it's likely your gearing, and its probably best to take it to your local bike shop for repair.