I'm happy to see them really committing to this. I didn't think I would be all that interested, but I will sure give it a go. Personally I wouldn't mind quality of life improvements and most of my friend don't really care for a 100% authentic vanilla experience but they want the old zones and world, a game world pre-Cataclysm but have no requirement for it being "authentic" to World of Warcraft Patch 1.12.2.
Each battle that you win will yield experience for each pet that participated. However, deceased companions will not receive any experience (so make an effort to keep them alive by swaping in a different pet). Experience gains are based on your pet's level compared to the wild pet. Defeating a higher level opponent will grant more experience than a much lower level one, but be ready for a tough fight!