How do tomatillos differ in taste from under-ripe tomatoes?
Under-ripe tomatoes definitely seem to me like they weren't exactly meant to be eaten like that. There are certainly still things one can do with them, but the flavor isn't always what you'd like (see tastefive's comment); they were selected for their characteristics when ripe. Tomatillos, on the other hand, have always been used like this. They're supposed to be tart and bright green (unless they're a differently-colored variety). I think tastefive was pretty on-target with the mention of lemon juice; they have a nice bright tartness to them but definitely no bitterness. They're quite pleasant to eat fresh, if you're the kind of person who likes tomatoes and fruit. (I just ate one to make sure I wasn't lying to you.)

A much bigger difference, though, isn't taste: tomatillos aren't really juicy. You can slice and dice all you want and your cutting board will still be dry. This is pretty clear from pictures - here's one I found with Google image search:



This makes them very well-suited for the kinds of Mexican sauces they're traditionally used in, more so than a green tomato would be.