I thought this was a silly decision by Stern. This was one of the best packages the Hornets have been offered for Paul, and it really wasn't that great for the Lakers, so he certainly didn't nix it because it ruined competitive balance.
Yes, Paul is the best player in the deal right now, which is generally the key...but, first, a great ball-dominant point guard has never been the ideal running mate for Kobe Bryant, second, the Lakers lost a lot of length which was one of their strengths and, third, they lost a ton of depth. It's extremely arguable that the Lakers would have been a better team if this trade had gone through. And, of course, the elephant in the room is Paul's knees, which are on the same general path as Roy's. He may not suffer the same catastrophic result as Roy did, but it's a huge risk factor.
Stern almost certainly nixed it because of the over-emotional but under-logical pressure from the other owners. Yet, this is no way changes the way the NBA does business going forward (as some are claiming in this thread). Stern was able to nix this because the NBA "owns" the Hornets. Unless the NBA is going to start holding other NBA teams in trust, stars going to big markets won't be stopped in the future. So, ultimately, this was a meaningless attempt to grandstand by Stern.
I don't really care. I don't think the deal would have materially changed the landscape of the NBA if it had gone through and I don't think Stern's decision will have any consequences to future deal-making. It's completely inconsequential (though amusing in the furor it's generated) but still a silly thing to do.