So, if you're curious...
Regarding Mounts Bought Before Cost Changes
If you were a low level toon who, before 2.0, had enough money to buy a mount a few levels before you could train for the riding ability... and then the "change" happened in terms of costs... you don't get double penalized.
To be more clear:
At lvl 32, I thought I'd buy my mount for Valdy, so I didn't have to worry about saving or re-earning the 72g or something it cost me... but then I took a break and when I came back, the costs had switched around (it originally was that the mount was costly but the training was cheap, now the training is costly and the mount itself is cheap). I didn't know as I approached 40 whether I'd have to pay the high cost for training after having paid the high cost for the mount originally. Thankfully, when I hit 40 and went to the riding trainer, I found I had "already learned" the riding skill for lvl 40. Essentially, I got it free because I had spent the big cash earlier on the mount, when it was still expensive.
Also...
Regarding Quests Completed Before The Expansion, and Turned In Afterwards
You get experience for them. I'm running around now turning in a few quests I had done while I was lvl 60 and that was still the max level, and I'm definitely getting experience.
Yay! Thank You Blizzard! It's the little things that bring big joy ;)