Define top of the line? $500, $1000, $10000
Same as in still camera's. Pixels, optical zoom, compression types, light sensitivity, battery life and available storage. Plus with video cam's the sensitivity and noise immunity of the microphone is important too. Optics quality is a little less important than with still cameras but the light sensitivity is more important with moving targets and no flash to deal with.
-Storage means a HD or DVD burner, flash isn't big enough. Burners you can drop in new blanks, HD your stuck until you can upload.
-Zoom 10x optical minimum as high as 25x. With 10x or more you want an image stabilizer feature on video. Actually, for top end you want image stabilizer and >10x period! Motion is funky to deal with.
-Batteries. Lithium will have more life 2x but are expensive 10x and have the annoying habit of bursting into flame.
- Any compression will pixellate the image. The faster the motion the higher the blockiness. Recording a lossless video gets big though. get the latest DVI, AVI or MPEG format. AVI is probably better. Multiple types would be nice so you can choose quality vs duration.
-You want a noise canceling, directional microphone. Wind hiss is a pain outdoors.
-Light sensitivty you'll have to compare the ratings on different cameras. It's been a long time since I did this stuff and I forget.
That ought to get you started...
When you think you're getting close hit some photography sites for camera reviews.