I don't suffer from this that much, as I'm not a huge collector of mounts, pets, or anything for that matter. I only have about 70 mounts, and that's a very ambitious estimate indeed! For those of us who are serious collectors, pet and mount panes new and old suffered from a serious problem: a lack of searchability!
Last night, in a Twitter conversation with Community Manager Zarhym, Lead Encounter Designer Cory "Mumper" Stockton said that a search function for mounts is "en-route for 5.1". Even with my measly 70 mounts and even fewer pets, I'm rejoicing. And, of course, glaring at the Abyssal Seahorse that mocks me from the top of my mount list.
What else could Blizzard do to improve the basic UI? I'd love to see a similar search added to the spellbook. My shaman was very confused indeed at the loss of Earth Shield, and eventually discovered that it had been taken away from her until level 26. A search could have made it far easier to find that! And I'd love to see a favorites section in the mount and pet journals. Mostly to get rid of that pesky seahorse.
[Edit: Thanks to all readers who pointed out that the pet pane is searchable already! I must write pieces immediately after titles so I don't forget key details!]