DC has some sketchy and downright dangerous neighborhoods. You want a hotel where you'll feel safe and comfortable.

Remember that DC is very hot and humid throughout the summer months. If you're going then, walking can be unpleasant, so think about the logistics of getting to what you want to see.

Around cherry blossom time (roughly March to early April), it can be extremely difficult to get a hotel room and the available space is very expensive. Major school vacation periods can also be bad as many school kids do (somewhat educational) trips to DC.