The #1 upgrade for a druid is a 20 slot bag. Because we can play four different roles we like to accumulate all kinds of gear; tanking, melee dps, healing, spell dps. Of course I've never rolled for gear I'm not going to use, but I've done quite well for myself picking up the scraps no one else wanted.
That lesson particularly hit home for me a few months back when I made the big switch from Moonkin to Feral. I'd been doing Karazhan and Gruul for months as a chicken, totally clueless about feral mechanics. Fortunately I'd been squirrelling things away in my bank so that when I made the switch I already had about half the gear I needed.
IMHO every druid should have a healing set. Always. I haven't actually used mine in months, but it makes me feel good to know I can get to +1500 heal and 200 mp5 when casting if the situation calls for it. Druids can heal, you never know when it might come in handy. In addition I strongly recommend healing druids make a cat DPS set; just store any fancy items with lots of +agility, +attack power, or +hit. It'll make your quest grinding easier even if you never respec.
Beyond that, it does pay to keep gear around that you might need some day. But it gets a bit messy. I just spent an hour and a half clearing 30 bank slots. I should spend so much time cleaning my house!