Mike Neal is a good example of a 3/4 DE who was a DT in college. Tyson Jackson (KC) was a DE at LSU and was drafted high in the first round in 2009 (#3 I think), but he has been pretty much a bust. The point with Jackson is 3/4 teams are willing to grab them earlier than they should maybe go because they are tough to find.
Typically the better ones have a large wingspan for batting down passes and can play the run really well.
Well the difference between Neal and Jackson is Neal was a DT in college and Jackson was a DE. If you want to target a good DE for the 3/4 keep an eye on the following guys:
DE's
Cameron Jordan - Cal
Adrian Clayborn - Iowa
Cameron Heyward - Ohio State
Allen Bailey - Miami
Christian Ballard - Iowa
Pernell McPhee - Miss State
Lazarius Levingston - LSU
DT's
Stephen Paea - Oregon
Drake Nevis - LSU
Marvin Austin - UNC
Jarvis Jenkins - Clemson
Sione Fua - Stanford
Ian Williams - Notre Dame
Colby Whitlock - Texas Tech
Mike Blanc - Auburn
Cory Grant - TCU
Anthony Gray - Southern Miss
Ollie Ogbu - Penn State
Martin Parker - Richmond
Ladi Ajiboye - South Carolina
Ted Laurent - Mississippi