Besides the cosmetic differences that you mentioned before, the 84 has a similar platform to the 85-87s. That is the structure of suspension, brakes, spindles,a- arms, and uprights are pretty similar. The biggest difference is the wiring harness and transmissions/engine choices. But if what you are looking at already has a v8 in it, someone has already taken care the biggest issue with an 84 engine swap. So this would not be an issue. If it has a nice firebird interior then you probably would not notice a difference in interior options either.
I say if it is clean you should go for it. Any of the upgrades that you could do for an 85-87 with a V8 you could do with an 84.
I dont think I have overlooked anything. (Then again I am a little 84 biased
edit to add: the 2nd gen headlights are nicer on the 86-88 (i think) but they can also be used in earlier models with little modification.
1984 Base Coupe Red Stock MY8
1984 SE Black Stock
2002 Grand Prix GTP 40th Anniv. Stock
[This message has been edited by lurtz (edited 03-14-2011).]