The talent trees aren't going to exist in their current form. You will choose a spec at lvl 10. Say, Shadow Priest. You will get all the abilities integral to Shadow automatically as you level.

The six talents you get every 15 levels are utilities, as outlined above. Some are better for PvP, some in raids, some for soloing. Some heal, some damage, some crowd control, etc. They will be as easily interchangeable as glyphs. Which has been spelled out in not only this post, but multiple others about the new MoP talent system.

And as for flying, this isn't a new concept either, but there are people new to the game with Cataclysm. We couldn't fly until lvl 70, and even then only in Outland, not in Classic zones. When we got to Northrend, we couldn't fly there until lvl 78, and still couldn't fly in Classic zones. Partway through Wrath they fixed it so lvl 60s could fly in Outland right away, alts could get a consumable heirloom book that let them fly in Northrend...and still no flight in Classic (due to graphic limits and how the world was built). Flying in Classic was adding at the beginning of Cata with the changes and updates to the world.

Being able to fly right away in Cataclysm zones? Eh. it wasn't that challenging, honestly. I mean, my lvl 60 Hunter has explored most of Cata's zones now and earned a ton of experience (and candy) from that, just flying over everything. Exploration has little to no meaning, and quests are easily cheesed at level.

So not being able to fly in Pandaria until lvl 90? I don't mind. I can fly everywhere else, and I have a ton of ground mounts that will love to get out of the stable, anyway.