The restriction on summoning in Netherstorm is going to be removed entirely! Hooray! While this change won't vastly alter gameplay, it's another nice little convenience thing. Those are usually the most enjoyable changes, I think. This far into the game, there really isn't much need to keep Tempest Keep restricted to those with flying mounts. Plus, as all of the level 68 Druids with their Flight Form have displayed, you don't need to be top level to do some damage in most of Tempest Keep either.
This seems like it might be another change with the intent of opening the game up to more players and speeding up the content to prepare for Wrath of the Lich King, but most people I know never liked the summoning restriction anyway. I, for one, am glad to see it go.