Airsoft is neutered paintball. I play/played, but can't get an adrenaline rush from it. Even if some guy is 5 feet away, there's no fear that it's going to really freakin' hurt if I make a mistake. That, and almost all of the airsoft players I've run into just don't have anywhere near the aggression level of paintball players

Stay away from the airsoft "paintball" pellets. They will shred in your gears and ruin the rest of your day (and the next several as you attempt to clean in it, reassemble it, and pray it works)
Just like paintball, 95% of the time, the person will call themselves out when you shoot them... if they feel it, which you often don't, 6mm or 68cal. The other 5% you have to shoot until they can't handle the pain anymore. My personal rule is to shoot them until they call themselves out. If the first one doesn't do it for them, 14 rps gets them to change their mind fast. Works almost everytime. Sometimes they run and you have to chase them, shooting them in the back and screaming that they're a worthless, n00b, cheating potato.
Airsoft guns shoot too flat

Most like it.. I even do to a certain extent.. but I think it's very limiting. Since concealment is just as good as solid cover in both, paintball guns can let you drop shots and shoot around corners in a way airsoft guns simply can't.
If you want aggressive airsoft, you have to go to Japan. Unfortunately, all of the non-paintball airsoft players we've run into have this really bad idea stuck in their head... they want to be "tactical". Got a useless silencer and laser sight? That's tactical. Got a helmet on? That's tactical. Carrying an extra 30 pounds of crap that you're never, ever gonna use? That's tactical. Got a 10x scope for those scary very long range 50 yard engagements... that's tactical. Cornering 10 airsoft armed paintball players in a 40 on 10 situation and still having your flank rolled in 2 minutes... that's reality.
Sub... here's a drill Swarm (from the MA) taught me for those shots where they're just hanging out alittle bit. Say... the very edge of an elbow, heel, or toe is sticking out from their bunker...
Find yourself a friend with a stopwatch, and a good bunker, with your target bunker however far away you're comfortable with. Tape a quarter up at the target bunker. Just hanging off the side, on the bunker, doesn't really matter, as long as you have a quarter sized target. Tuck into your bunker, and post up on the quarter. Sit there tucked up and wait up to 5 minutes. When the command is given (it's kinda random to keep you on your toes), 1-shot the quarter. If you miss, get comfy, you get to do the fun wait part again.
Next, find yourself a telephone poll, tree.. whatever.. and tape up a row of paper plates about chest height. Back up to where you're comfortable, and begin walking in a circle and shooting the plates. Gradually pick up your pace. After awhile of this drill, you should be able to put all your shots into the plates at a dead run. Personally, this is what I'm really bad at. However, people that master this drill get very, very dangerous on the field.
Only problem with airsoft and paintball is that they're so far from realistic... it's gonna take Simunitions to really make it believable.