Some great cards have been spoiled from the upcoming set (Journey into Nyx)
I want to mention one in particular, a common
This is extremely powerful, for a couple of reasons. The obvious is that there are a lot of enchantments in this set, the gods, the man-auras, etc. Secondly, green doesn't get enough of this type of card: straight card draw. For an enchantment deck (very popular, I have a blazing fun one with Kor Spiritdancer, rancors, etc, that this would fit perfectly into). Thirdly, three cards get sent your graveyard to boot, a la mulch. I doubt these will fetch more than $0.25 for a while, but I would grab as many foil versions as you can while they remain plentiful and cheap.
This also grabbed my attention:
Wizards has been playing off the Juzam Djinn since its printing, with cards like Grinning Demon, etc. But this is a FAT incarnation of that theme. 3 mana takes it down to a first-turn dark-ritual, and some decks rely on opponents drawing cards (Consecrated Sphinx, Underworld Dreams, Psychosis Crawler, etc).
It's a card that needs to be pretty perfectly balanced to both see play and be unbroken, and I think wizards may have dropped a masterpiece here. I wouldn't expect this to explode in price for a while, if at all, as its utility is probably in Legacy or Modern, if anywhere. But it definitely caught my attention. I will be buying some if they fall, post-release, to below a dollar.
Now to the good stuff. Exploding Cards:
I just dropped $50 on 5x FOIL copies + 12x non-foil copies of Nath. You can see the classic momentary swells (eBay [golden] between march 8->March 16, and Amazon (light blue), both out-of-stock regularly, it seems to have "caught" at close to $3.00. I've seen legendary creatures from before Commander was a format percolate like this for a few weeks or months, then explode (examples: norin the wary, sygg river cutthroat, zur the enchanter...). There are only a handful of foils left, and they're about to go over $10 apiece. I'd expect to see foil versions at $20 apiece in the next few weeks if these indicators prove true.
Shadowmoor and Eventide's Filter Lands (Fetid Heath, Mytic Gate, Sunken Ruins, etc) have been surging for the last couple of weeks. I could see these 'never looking back' and going the way of fetch lands, up to $30-$40 within this year. I have 5 total now, if I had 1-year investment money right now they would go to these. Some examples:
Tamiyo has been on my radar for several months now, I think I need to jump in now with a playset if I want to own these. I don't see Wizards reprinting her. (Note: view is 16 weeks, not typical 8 weeks)