I'll also send forth some love for Runebound. It should, I think, be manageable by an interested 8-year-old. I haven't tried it with my 6-year-old, although she's been "counter helper" before. FWIW, Runebound also plays well solo, which is important considering that I have no heavish-boardgame friends at the moment. As someone else said, there are a *lot* of expansions, 6 more on the way this summer, and the cheap decks add a lot of interesting twists.
I've had my eye on Descent since before it came out, and I've toyed with the idea of buying it then developing a simplified rule set for my kids and I to use. I really like the Runebound+Descent overland/dungeon idea someone mentioned, too.