hijackable kernel or at least upgradeable OS

I think that's the most important!

Seriously though, you're right. Most microwaves UIs are still in the relative dark ages. My microwave clock is rarely set correct. Brownouts are so common that it often loses time. Usually just after I've resigned myself to setting it again. Give me an ethernet port and a linux based OS, and I'll use ntpdate to keep it sync'd to my server.