This is something I have noticed lately and maybe its just me personally and not something that is true of everyone but when I watch videos on youtube (480) or watch a sd movie on my hd monitor (fullscreen) it doesn't look nearly as bad as if I ran a game at 480p which will have terrible jaggies. Not to say there aren't any jaggies on the videos they just seem more smooth and less noticeable. The only reason I can think of is that since I am running the video on my computer it is being upscaled by my graphics card or the video player that has better upscaling and when I am playing games at a lower resolution it is my monitor that has to do the upscaling which it might not be so good at. Anyway maybe someone with more knowledge than me can help me out or just tell me its all in my head .
edit: I was just playing around with my graphics card and it wasn't even set to affect internet playback so it isn't my graphics card I don't think.
Videos are usually effectively captured from much higher resolution sources (reality or drawings, depending on whether the video is live action or animated), and the camera automatically picks up the average brightness for each pixel. Games, on the other hand, generate the scenes on the fly, so they either use the brightness for the center of each pixel (AA off) or average the brightnesses of a limited number of points for each pixel (AA on).