I have played the DLC and it was an improvement. That still doesn't mean that GTA Vanilla lacked a lot of the wackiness that made GTA: Vice City and GTA: San Andreas so fun to play.
One of my big gripes with GTA IV was. . .what is there to do with all this money I am earning? There were no businesses to buy and manage, no territory to keep mine etc.
There was a shortage of cool stuff to do really. No Jetpacks. One of my favourite things to do was fly as quickly as I could over the military base in GTA:SA, parachute out, land it, and see how long I could last. The missions were also less interesting in IV. Tracking helicopters over highways, burning a guys weed plantation down with a flamethrower, getting in a lift with a bike and jumping between buildings, distributing porn flyers from a plane etc were all fun and GTA IV lacked that stuff.
Character customisation was also missing. I enjoyed buying different clothes, getting different hairstyles, tats, going to the gym, getting fat etc.
Then there was my annoying 'friends' that rang me far too often and drove me completely nuts.