Two Stones have confirmed identities: Tesseract (Space, confirmed by Feige), and The Orb from GOTG (Power, directly identified by The Collector). The Aether's power is still pretty vague; there's been no confirmation that Loki's Spear contains the Mind Gem; and that leaves 2-3 still unaccounted for.
*Loki's Spear: yes, it's a popular theory that this is the Mind Gem. But a closer inspection reveals that this Spear isn't nearly as powerful as some people (including Von Strucker) think it is. As it stands right now, the Spear only seems to work if it touches a person's heart (strangely enough), and explains why Tony's artificial one was immune to the touch. At Stuttgart, the whole "kneel!" scene showed that no one there was under it (or Loki's spell), and merely knelt out of fear for their lives; even Cap and a Random Old Man didn't even go that far.
Otherwise, its only other displayed power was enraging people within its presence --- which was pretty much all Loki was aiming to do by causing Banner to Hulk out. It would also tend to explain what's going on in the CATWS post-credits --- Wanda & Pietro are spamming their powers because their rage is being controlled by The Spear. But even that issn't a particularly powerful "spell": most of The Avengers quickly figured out that the Spear was causing their petty infighting, and calmed down (except Bruce, of course).
So, if The Spear houses an Infinity Stone, it's a piss-poor cosmic artifact that doesn't have much power or range. I'd hope the Mind Stone is shown to be a *lot* more powerful than that.
*Aether: the only thing we know about it is that it controlled Jane Foster's soul, and drained and nearly killed her; and seemed to control Malekith's soul, and made him uber for a brief time. So maybe it's the Soul Gem?
If so, that leaves Mind, Time, and Reality.
My spec, on two of them:
*Reality: what better place to introduce this one than the Dr. Strange movie? Because what is magic, if not altering reality?
*Mind: Ultron, believe it or not. Dunno how the Stone would wind up in a killer robot on Earth, but it *would* be a great excuse to explain how a robot that's apparently made by human hands would suddenly develop supaa-genius intelligence and start hyper-upgrading. Hell, even HAL 9000 was ultimately shown to have been guided by cosmic forces. Plus, rumors have just in the past 24 hours surfaced that Josh Brolin as Thanos will have a bigger part in AOU than he does in GOTG. Chasing down an Infinity Stone --- or inserting one into the equation against Earth's Mightiest Heroes --- would give Big Daddy Titan a reason to be here. Plus: Kirby crackle. I'm just sayin'.
*Which leaves Time, for which I don't have a clue. My best guess for that would involve time travel, of course, which instantly brings Kang to mind; but I have no idea when and where we'll first see Kang in the MCU, if ever.