The D70 is 6MP cam instead of a 10MP one. That's a pretty notable difference.
The D70 takes compact flash cards, the D80 takes SD cards. Compact flash are a hair more "risky" because they have pins, so you generally want to avoid popping and re-seating them a lot to avoid the risk of a pulled pin. SD cards are obviously smaller, and a hair more expensive per meg.
Someone commented on the max storage capacity on the D70, which is presumably true, but keep in mind at 4MP less, you won't need the high capacity quite as much. 2 gig on a D70 in RAW will get you like 200+ pictures. In JPEG/fine you're probably pushing nearly 500... maybe more. If I remember I'll flip my D100 into JPEG/fine when I get home. It has a 1 gig card in it. I think it reports like 265.
The D70 tends to take sharper pictures right off the bat because it has a less noticable anti-aliasing filter and the sharpening is turned up a bit. This isn't necessarily a big deal except it's harder to "unsharpen" an image than it is to sharpen one.
ummm... I think the D80 has a bigger LCD.
The D80 is obviously going to be newer/faster. The D70 is based off of the D100 which was the first entrant into the prosumer market, but the D70 is SOOOO much more camera than the D100 was in so many ways... it's really a good camera.
BTW, if you go for a D70, make sure you get a D70S... it's notably better than the D70. Faster, bigger screen, etc.
If you're money concious, then I'd say find a GOOD D70S and go that route. Otherwise, get the D80. You really can't go wrong either way.