I think part of it has to do with the ease of obtaining gold over the years. Back in vanilla, it was an incredibly long time before you saw that first gold piece. Saving up for an epic mount at level 60 was almost a rite of passage. Hardly anyone I knew had enough gold to get one when they actually hit level 60; they had to work at grinding enough to afford the mount. And racial mounts were racial mounts. You couldn't get a horse if you weren't human, and you couldn't get a raptor if you weren't a troll. When that changed, it opened up opportunities to actually start collecting -- and when gold started getting easier to obtain, especially through the addition of daily quests, the means to collect got substantially easier.
As for pets, trying to collect pets was an exercise in how well you could manage your bank before that remarkably handy pet tab existed. In fact, I think that's when I started collecting both in earnest -- once I didn't have to worry about bank space, it was easy enough to nab every pet I ever wanted. With the introduction of achievements, I began nabbing even those pets I wasn't particularly thrilled about. As for armor sets -- well, they're a badge of honor, right? Plus, they're pretty.
But that's not all I collect. I have a full collection of Steamy Romance Novels, just for giggles, and a bunch of gray items that have no real purpose. Heck, my rogue still has her Thieves' Tools, even though they aren't needed anymore. I guess it's either nostalgia or some latent urge to squirrel things away. It carried over into Skyrim, where I obsessively store every book I come across -- not for any real reason but just because I like the thought of having a library, you know?
Are you a collector? Do you like squirreling things away for a rainy day? Do you gather mounts and pets obsessively? Has transmogrification turned into an excuse to grab all that gear you never thought about keeping before?