Hello, I just figured this would be one of the best places to stop by to look for an answer. Recently I've made an avi in 3DS max and tried importing it into AE. Here's the problem: Prior to importing it, the video looks amazing. It's crisp and unpixelated and just looks plain good. After importing it however... well it looks like crap to be honest. It's utterly pixelated and it just destroys the video quality. The aspect ratio is the same and the size of the composition is the same... I just can't figure out what the problem is at all.
Need a little more detail. What codec? What size? What PAR? If your footage is interpreted as non square pixels and you have Toggle Pixel Aspect Ratio checked in the Composition window you'll see interpolated pixels there. If your comp window isn't set to 100% magnification you're not seeing every pixel. If the Composition window is set to fit and the magnification is something like 63% then your preview is going to look bad. You may improve the look by going through the Previews section of preferences and setting the Zoom Quality to More Accurate.
Also, if After Effects has automatically interpreted your footage as interlaced, the footage will be deinterlaced during previews - basically cutting the resolution in half.