I used to do a bit. In terms of finding things of value, top of the list is probably finding lost excavator bucket teeth and pins a few years back when I worked in a quarry. Not quite as glamorous as buried treasure, but there we go.
I did go right over the ruins of an old farmhouse once. Found a couple coins, all early 20th century, but maybe 200 rusty nails, bits of wheel rim, rusted bits of farm machinery and plenty more unidentifiable scraps of metal to go along with them. Decided at that point it wasn't worth it!!
The key to finding really interesting things is prob much less about metal detecting than serious research beforehand. I reckon with some time spent in a local library or on the internet you could come up with some new places... roman paths, ancient settlements etc, that could probably yeild a huge amount of stuff. Then though of course there'd be the issue of getting permission to actually dig there...