So, I like KGoldrunner, but I have some thoughts:

I don't get why, when control is set to holding down the key, there's such an annoyingly long delay between when you press it and when the player actually moves. Actually, I don't get why this is the case ever. Is this a limitation of the library used, or is it some sort of weird design choice? If it's the latter, it's a bad design.

It would be very nice if it was possible to share levels easily. Right now, you have to do a lot of manual work if you plan on doing that, AFAIK. I think it would be pretty easy to write a script that "imports" and "exports" levels from and to portable level files. It could mostly just involve putting the levelset data and level files into a Tar or Zip archive and giving the archive a different extension (like "kgl" or something). I'm interested in doing this myself. Would it interest anybody else?

If you are looking at implementing something like that, I suggest asking on kde-games-devel@kde.org as they may be able to provide some helpful pointers, advice, etc.