Before 2.4.2 is even up on PTRs, MMO-Champion and World of Raids have undocumented changes listed on their sites. That's dedication! Of course, given that none of this can be verified, you should take it with a grain of salt, but most of the time changes found in the game files end up going live. Here's what's been noticed so far:

Most mount-speed-increasing trinkets (Carrot on a Stick, Skybreaker Whip, Riding Crop) have been changed to enchantments, which can be permanently applied to a single mount in your inventory and cause it to become soulbound. Furthermore, Charm of Swift Flight (which will stay a trinket) now also reduces the mana cost of Flight Form and Swift Flight Form by 115 mana, and Carrot on a Stick is now craftable (1x Simple Wood, 1x Fine Thread, 1x Golden Sansam). I imagine it would be an engineering recipe.

This seems like a good change for the most part, since we won't have to use add-ons to swap in our riding speed trinket all the time (and we'll save an inventory space). However, it does have the major drawback that you'll need to get a new one for each of your mounts, which could get expensive if you're a mount collector. More business for the leatherworkers, I suppose.

Continuing on the mount theme, a sort of horned Swift Zebra mount appears to have been added to the game, as well as a Headless Horseman's Mount (both pictured).
Aspect of the Viper has been buffed. Previously, it generated mana equal to up to 55% of the hunter's Intellect every five seconds; now, it generates up to 55% of the hunter's Int plus 35% of his or her level. This means a boost of up to 24.5 mp5 at level 70.
There are also some changes to various items and recipes; check MMO-Champion for those. What do you think of this mount trinket change? Oh, and if you haven't seen the official patch notes yet, check Adam's post from early this morning.