Basically just that the old system has the hero images all nicely fitting together in a reasonable space, so scanning your eyes over them is easier. With the new grid, the hero cards take up twice the space on a very wide swathe of your screen so you have to scan your eyes back and forth across it. Here's an image to illustrate the difference:
http://i.imgur.com/UdlEx.jpg
For me personally, I would far prefer a pick layout more similar to the old version, just because it seems so much clearer to me. And again, really feel the need to emphasize just how dark the hero portraits are in the new picker.