Short answer - "everything"...
I was bummed out to learn my '85 Suburban with a tailgate cant be converted to barn doors unless your an experienced body man--I had found a nice pair of doors with the same two tone paint & pinstripes at a salvage yard 4 miles away for $80 ,they were just picking the Suburban up with a forkloader to haul it towards the crusher when I saw it,--I yelled at the guy "drop it right here,I want those doors!"..
The guy in the office asked me what truck I got them off,and he said "OH,your lucky--we're crushing everything out in that section of the yard...".
He said "Your Suburban has barn doors huh"--I said "Nope,but I hope it soon will"..
He replied "well,far as I know,after 1978 or so,you cant just ditch the tail gate and bolt on barn doors any more--there isn't any spot for the hinges to bolt too ,and I think the floor is different too--on the older ones everything was there,and all you had to do was bolt them on"..
I bought them anyway,and he said "no cash refunds you know,in case you cant use them"...
I went home and looked the truck over closely and sure enough,the floor was a lot different on the donor truck than mine,where the doors seal against the floor--I learned after doing some googling that the barn door trucks are a whole different body than tailgate versions..though it can be done,you'll need to hack out the rear part of the barn door burb's floor and graft it to yours,and weld the hinges on ,because there is no reinforcement plates in the body there to drill and tap into..I suck at body work,so that option was "out"..
I ended up bringing the doors back a week later--the guy who sold them to me said "well--normally we don't give cash back--but I had a guy come in looking for these doors a day after you bought them!--he called the guy,he had left his phone # with him,said to let him know if he got another barn door burb in--he still wanted them,so he did give me my money back--$75 of it anyways,all he had on him at the time..
I wish GM hadn't changed things around after 1978...I think it was a dumb move..I hate tailgates with power or crank up windows that are nothing but a headache..not being able to put the barn doors on my burb was depressing,after that I was not very enthused about fixing it up and registering it,so it has sat since I got it in November of 2009..
After being parked that long its rotted the rockers and the list of its other woes keeps growing,so I'm not sure what I'll end up doing with it..I did put a Fisher plow frame and a Diamond plow on it,got it operational,plowed my driveway once with it to test it out--then parked it..