For those of you unaware, one of my favorite aspects of the game today aside from gold and GDKP raiding is pet battles. I got into it last year and like most things I ended up documenting certain things. Maybe this will be of use to you in case you're trying to figure out how the tamers work and why people do the quests everyday.
The reason is simple: You get a lot of experience depending on the level of pet you are carrying. I had a theory when I first started that the experience had some sort of diminishing return, so this led to me actually performing battles with every single variety and documenting the results.
As you can see from the chart below, the yellow highlight shows you the spot where you will earn the most experience, and it's a mirror. This is why it is bad to use a level 3 against a 25, your best return will be a 10 or 11.
Note that the chart assumes you are wearing the safari hat. If you aren't, then you net 10% off that amount. If you are using the pet treats, obviously you'll account for the extra boost of 25%. More or less I use this as a guide so I don't waste experience on pets that are too high or too low. The difference between 25N and 25P here: 25P are Pandaria trainers, Ns are Northrend and Cata.
Additionally, I put together something for 1-25 Wild Pet battles, and since 1-19 is mostly boring for folks I'm only showing the returns you see when fighting level 20 to 25s.
Again, yellow shows the sweet spot and assumes the safari hat is worn. Where I placed the "x" means I never got to those levels, but you get the idea.
I figure that since we're about 2.5 months out, I can share this little nugget with you in case you're looking for something to do. I'm presently trying to get all the pets I have to 25 (or at least most of them) prior to the release. I have 550ish pets, 170ish at 25, and the vast, vast majority are between 16 and 24 right now. Only greens are 11-15. I figure if I get this out of the way, I won't need to bother with my OCD for about another year.
Hope you have a great day and thanks for stopping in!