Lense focus and clarity
color temperature and gamut
color saturation
exposure
and a host of other things. Differences in some aspects of the image can be compensated for in the video editing process. However, if the video is live, additional hardware will probably be needed to adjust the output from each camera to achieve a match.
As with all cameras, it's a lot easier if you get it right in the camera, but low cost equipment does not usually offer the necessary manual controls.
I have a Panasonic PV-GS65 and want an additional camcorder for my video shoots. What aspects do I need to be aware of when choosing my second camcorder? I have been hard pressed to find another GS65 (budget, budget, budget) so I am wondering how close to the GS65 specs do I need to be and what would the viewer notice with two different camera outputs? I intend to broadcast on internet and community TV so my quality standards aren't blockbuster. This is my first query regarding this, so any knowledge/advice is welcome. Cheers, justjacks.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic