Regular flight is a rare occurrence in most of my games (actually, mounted anything is a rare occurrence). When I ran Age of Worms for the first time back in...2006, maybe 2007, the players eventually bought a hippogriff during one of their brief stays in Sharn (which the bard quickly made into an Improved Familiar).
It was pretty cool despite being able to only carry one person at a time, as everyone was loaded down in heavy armor, but the novelty wore off when the human warblade picked up a Siberys dragonmark that let him use greater teleport once per day. Also, I think the cleric could cast fly on everyone. Maybe they will rein that sort of stuff in this time around, thereby making flying creatures and magic items a greater incentive?
Griffons and hippogriffs—both carnivorous quadrapeds—do not see much change. I guess griffons are native to mountains, so that is something else to differentiate them besides the Hit Dice and levels of previous editions. I found the bit about griffons enjoying the flesh of hippogriffs and unicorns to be an interesting addition: if nothing else it could add some conflict.
The pegasus sees a pretty drastic change, becoming a celestial spirit fused with mortal flesh. It also mentions them being gifts of good-aligned deities or, very rarely, appearing to serve good characters on their own. I can get behind this exalted origin, especially because it makes the answer to the evil-mount question obvious: nightmares.
I think that having the default wyvern weigh in at Huge is a bit much. If you are going with the "bargain-basement dragon" comparison, then why not give them size categories, too? Most wyverns run the range of Medium to Large, with truly ancient ones eventually growing to Huge. This is an instance where you could look to 3rd Edition's monster advancement, where at a certain point the monster's size increases. This would make it easier to handle multiple wyverns (or wyverns as part of larger encounters) and, well, some of us have wyvern minis.
Rocs rollback to their 3rd Edition size of Gargantuan. This I can get more behind, even though it sucks for my roc mini, as I do not expect to see many encounters featuring more than one (though one being ridden by a storm giant does sound awesome).