i think it would be interesting, but would make farming too easy if you got a bonus drop rate. could you imagine farming snurble tufts and fleece? megalocrab legs? it wouldnt help the market for them, but it would deminish their value. they might not even sell after so long because you would get more with just a little bit of time.

aside from the +drop rate, i think it would be a great idea, getting an achievement at 25%, 50%, 75%, then 100% in the main portion(what we have now), but that would be flawed by expansions, so it would be best to have it by area. this way there would be no conflict with 100% completion, then having an area added and keeping that 100%.