I can't answer on behalf of how fun feral is, but seeing as all the gear dropped using the dungeon guide gear filter is the same for feral and guardian, yes, you can tank using the dps gear. You will however want to change trinkets to tank ones instead of DPS, and you will also have to use the Guardian spec instead of the Feral spec. In case you didn't know, Kitty dps and Tank bear has their own 2 "trees" now, giving druids a total of 4 specialization. Raiding wise I would strongly recommend having a tanking gemmed and enchanted set. For starter heroics I'd also go for the tanking enchanting/gems to begin with, not to cause too much of a drag on the team.
I am having a LOT of fun with my feral Always enjoyed this spec, now I find it even more fun with different talent choices, symbiosis, and HotW talent is awesome if needed some burst healing, ranged dps or tanking in 'OH #*$%!' situations. Add to that an awesome dps if you know how to play it properly - its great. Overall I enjoy it very much.
About tanking in feral gear - yes, but like someone above said - you'll have to have trinkets and spec changed to guardian to not be so squishy
have tanked and DPS 3/6 normal and you can use exactly the same gear for both - gearing for hit/exp cap then mastery/crit will give you near-ideal secondary stats for both roles as well (depending on playstyle of course)
Feral is amazing right now HotW makes us such a fun class to play.
I've takend all the HC's in full DPS gear (in bear spec ofc) and healers have said it was easy to heal me and i hardly too much damage. All i'd have to do to tank HC raids is change my meta and forge to crit/dodge.
Feral seems fun again and MoP seems much better than Cataclysm so far.
I should qualify that it took the game quite a while to get here and I work 60 hours a week so I have a lot of catching up to do. As for tanking in cat gear you will be quite low on dodge and stamina. questing or regular 5 mans should not be a problem, I would be cautious about anything else.
Atm with the gear I have (ilvl 476), I'm almost Hit and Expertise capped. I have about 472 dps gear and I can tank in it, if I have a decent healer. Just have to self heal a lot more, since I don't have the Dodge from my tanking gear.
But I would suggest just reforging into Dodge, it's not like it takes more than a few mins and cost of it is earned after a dungeon - more or less. So I don't know, why you'd actually wanna go tank with dps gear. Sure you will do more dps but with Vengeance, no tank should be ashamed of their dps - nor should it be needed to complete a dungeon.