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?