The multiplayer is kickass, you just probably suck at it. You miss the planes? to be honest i feel planes would be pointless in this game (i did enjoy them in SA) you would fly accross the map in 10 seconds and then what? ok you could fly over the ocean but that don't really do it for me. You could fight police helicopters. You got all the acheivements? you could try for that. Practice your skills on the bikes you can do some pretty fancy stunts. Buy every item of clothing. Though for me the thing i enjoy most now (even though i havent got 100% but have completed story) is multiplayer, i think it is brilliant. If it bores you try a different mode, you will be surprised how enjoyable some modes can be given your on a team with good players.
Edit* I forgot to say what i think about it I think this is the best gta game yet, though i havent had as much fun on this one as i did on gta 3 (the story missions there were the best of any gta game imo). The new graphics and physics make for a great improvement to gta. But in doing this rockstar will have annoyed people. (though i cant help but feel if it had been the same as previous titles with new graphics a few of these complainers would be arguing it isn't next gen or something along them lines)