According to a couple of reports, YouTube has started to compress the videos files we uploaded to them in higher resolution (better quality), but the catch is that you have to add an "&fmt=18" tag at the end of the video's URL to see the higher-quality version.
A comparison --
#1 My video "Slave to Love" as shown on YouTube in low resolution:
#2 The same video with "&fmt=18" added at the end:
The improvement is not dramatic, but if you watch closely, the difference is visible, especially the title in the beginning, which looks sharper.
Another comparison --
#1 "Love the One You're With" in the typical YouTube low resolution:
#2 The same video in higher resolution:
The improvement is clearer in this case, at least the way I see it.