The trick to writing a good interpreter is not in the writing of it; it's in the part wherein you manage to completely avoid making any decisions you might regret in 10 years.
Oh, sure. :) But a lot of people seem to find them impossibly intimidating, one of those things that are "perceived to be magical artifacts, carefully crafted by the wizards, and unfathomable by the mere mortals."* They're really not that tricky at an implementation level, until/unless you're going for extremely high performance.
Keeping reverse compatibility is a huge pain, though. No argument there!
