Yup, what Kevin said. Late spring you will get the biggest waterfall action, but the higher elevations can remain closed until June. By the the valley floor is an unpleasant madhouse. Winter is great, but you won't get any waterfalls really, and fall has awesome color with all the high passes still open, but mostly no water.
If you look hard you can see a lone hiker on the bridge over the top of Nevada Fall. I made this same hike from the valley floor many years ago when I was a healthy youngster. Oh to have had digital back then!
Lonely at the Top by
lennycarl08, on Flickr