Keeping in mind that money makes owning multiple platforms extremely difficult for me, and I'm unlikely to buy any more than two consoles in a given generation, here are my thoughts...
Xbox One: I'm not exactly sure what I'm waiting for at this point. I have the money. I think I'm just waiting for info on when Project Spark and the first wave of self-published games are going to hit the Xbox Games Store. When I get my Xbox One (which I anticipate will be sometime in the next couple months) I plan on trying out Killer Instinct for free (hoping the dojo mode can do a good job of introducing me to the complexity of fighting games) , and maybe buying an self-published game or two (Divekick has caught my eye, I like that you can play it without having to master a whole bunch of button combos). I am super excited for the lineup of AAA games coming out for the Xbox One this year, games like Destiny, Halo 5, Sunset Overdrive, The Witcher 3, and The Division.
PS4: I'm mainly waiting to see what Naughty Dog (and to a lesser extent, Sony Santa Monica) is going to do with the hardware. Right now, there just aren't many exclusives on the PS4 that I want to play. Transistor is the main PS4 exclusive I want right now, and there is a good chance that it could come to Xbox One after the timed PS4 console exclusivity runs out. But this could become a very desirable console if it can get some compelling exclusives, and I expect there will be eventually. If there's any console that is most likely to sit next to my Xbox One at some point in this generation, it is the PS4.
Wii U: I would need to see some great SEGA games released exclusively for the platform, and so far I'm not seeing it. Sonic: Lost World didn't impress me when I played it at Sonic Boom, and the mixed reactions I've seen from Sonic fans only confirms that it is skippable. Smash Bros is fun, especially since I can play as Sonic, but by itself isn't enough.
3DS: Handhelds don't interest me that much, and even though the 3DS has a lot of games, not many of them interest me. Pokemon is a fantastic series, and the demo I played of Super Mario 3D World was fun, but neither come close to justifying the cost. Nothing else really grabs me.
Vita: The Vita doesn't have anything that I really want. Like, at all. Any platform needs to have compelling games, and for me, it has none.
One last thing to note: a platform exclusive Skies of Arcadia 2 is pretty much a trump card, and all but guarantees that I will buy the platform necessary to play it, at some point in time. No way would I miss out on a sequel to one of the greatest games of all time, unless it wound up being absolute, Sonic 06 levels of hot garbage.
