I love how everyone keeps saying how dumb Wan was for closing the portals.
Were I in his position I would've done the same thing. There's mild...Racism for lack of a better word the the current Korra days that turned to extreme violence (That Vaatu helped instigate but the Spirits Vs. Humans thing would've happened without him) in Wans time.
It wasn't stupid it was what at the time seemed right.
I wouldn't say Wan was a complete idiot, but I was shocked (and somewhat happy) to see that the writers chose not to portray him in a "perfect" light, despite being the first Avatar. It's similar to how I was glad about their handling of Aang's and Toph's parenting, which was by no means perfect. It makes all three characters seem more human.
The problem with Wan's solution is that the presentation gives viewers the impression that he viewed the separation of the spirit and physical planes as a permanent endeavor, as opposed to a temporary solution for the time being. This would be like saying that the solution to the Fire Nation conflict once Ozai was defeated was simply to put a giant wall around them, permanently.
We have had quite a few characters (especially Iroh) remark on the importance of balance between the four nations, but he's a perfect demonstration of how that should apply to the Spirit World as much as the material world. So I wouldn't say Wan was a moron at all, but I do think his logic in that instance was very much flawed.