YouTube’s Handbook says…
Best Formats for Uploading
YouTube can accept almost any video format for upload, but for most users we have found the following settings give the best results.
* Video Format: H.264, MPEG-2 or MPEG-4 preferred
* Aspect Ratio: Native aspect ratio without letterboxing (examples: 4:3, 16:9)
* Resolution: 640×360 (16:9) or 480×360 (4:3) recommended
* Audio Format: MP3 or AAC preferred
* Frames per second: 30
* Maximum length: 10 minutes (we recommend 2-3 minutes)
* Maximum file size: 1 GB
I found that uploading SD video in larger formats than recommended results in a long period of waiting while the video is being “processed.” I think it is being resized to 480×360. When I uploaded a video in the recommended size, it was immediately available and looked better than the same video uploaded in a larger format. So perhaps you want to try 640×360 with your HD videos (depending on what YouTube calls “HD”).