I'm not worried that higher resolutions are going to make the game look worse or mess up the layers in a level. People are playing beyond what a level, and the game, was designed for when using anything above 640x480, so it's not the levelmaker's fault but the player's own choice to sacrifice graphics for resolution (actually, for a standard 17" monitor, full screen 640x480 looks pixelated...a higher resolution could actually bring about the same pixel sharpness that windowmode does).

A bigger issue is that higher resolutions would ruin the carefully balanced gameplay of levels which people have designed to work for 640x480 and of course the simple fact that people can see you before you see them.

There's also a minor risk that people may start downrating existing levels/tilesets because they don't look good in 1024x768 full screen, but I think that's just them being unfair, not a fault of higher resolution. What if you are a 320x120 user? Will you be allowed to downrate a level's eyecandy because your resolution prevents you from seeing anything?

Higher resolutions would mean that different levels could be made, with a new style of gameplay, long-distance face-offs, among many other things. Its current disadvantages are crippling, however, and I don't think it should be released in such a free and un-limited fashion. If possible, a resolution changer should become a new version of JJ2, compatible only with itself and no other ones, and not just done by a simple value change in Jazz2.exe, which you can just revert back to 1.23/1.24 with a hexeditor.