I've been coding an applet using Java 1.4 under the (possibly/probably misguided) assumption that newer JRE's are backwards compatible, so my applet could run on a wider array of target systems without requiring users to update their JRE.

Is this a valid assumption? I'd like to use the new language features available in more modern versions, but not if it's going to force users to update their Java installation to run my applet.

Anybody have experience with compatibility issues with regard to applets?

Is this a valid assumption?

I think it is. 1.4 version programs should run on 1.6

Mac's 1.6 is not quite done yet, so definitely stick with 1.4 for compatibility. We had all kinds of trouble with this in a project I worked on last year - for a while, I was coding on a virtual Ubuntu install because I couldn't run the project on my mac, until we cleared out a bunch of 1.6 stuff. Very aggravating.

"Write one, run anywhere" is a nice mantra, but you should still check your work on a variety of platforms.