I've read a few responses and figured I'd throw my hat in the ring. Asik is a clear upgrade over Lopez in one extremely important area-Pick and Roll defense. Asik is one of if not THE best pick and roll defenders in the league and it is a big reason so many people love him. He is also a tremendous help defender.
here's a good article explaining some of his versatility on the defensive end
The part I want to highlight from that article is this:
Here, Tony Parker is trapped in the corner and Danny Green moves toward him for the escape pass. Tim Duncan sets an off-ball screen while this happens and Asik slides out of the paint and below the free throw line to help:
Green has nowhere to drive with Asik in position, so he passes to DeJuan Blair. Asik rotates back to the low post to help Patrick Patterson:
Blair passes out to Duncan below the opposite elbow. Jeremy Lin reacts by standing between Duncan and Parker -- this is known as "stunting" -- and is able to get away with it because Asik quickly recovers to Duncan after helping on Blair in the post.
Lin switches back to Parker and Asik is back in front of Duncan in time to challenge the shot and force a miss:
There are several other examples of his excellent defense in the article, another good one here:
One of the biggest drawbacks to Robin Lopez is he can only defend the rim area. If the opponent can draw him away from there it is easy pickings, either they get a basket or we have to foul to stop the layup. Asik would solve this issue, as well as bringing the same strengths as Lopez does while doing them all better.