Every once in a while an item will get removed from the loot tables, or an achievement will turn into a feat of strength. The most recent example would be the vanity items from the patch 5.3 event, as well as the Darkspear Revolutionary and Hordebreaker titles. Maybe you couldn't log in at the time, or maybe you didn't bother farming the 500 Kor'kron Stone for a Whole Body Shrinka and now you regret not having one (sorry to let you know, but it's awesome). Maybe you deleted another toy item long ago to make room for bag space without thinking Blizzard would ever come up with a storage solution for toys.
My worst offender in this case is the hydra hunter pet, Spirit of Atha, which I did have tamed before they removed the ability to do so. In my infinite short-sightedness, I abandoned it to make room for a spirit beast (we only had 4 stable slots back then). I still hope Blizzard will make the hydra family of pets tameable again some day because every time I see one (which is exceedingly rare) I am insanely jealous. Then there was that mail gear from the pre-Wrath launch event which I probably vendored for bag space. Who could have foresaw transmogrification back then? Ever since these incidents I've become a WoW hoarder and don't want to delete anything. The Warlords inventory improvements can't come soon enough.
What did you delete or procrastinate on that you can no longer get back?