By Sony and Microsoft being highly disappointing with their new consoles and hence handing them the win by default.

3DS
- Zelda
- Metroid
- SSB

Wii U
- Zelda
- Metroid
- Retro game
- REvelations port or sequel, or REmake 2
- SSB
- MK

I want everything.

The new Super Smash Bros. I haven't bought a WiiU yet. SSB could very well be the deciding factor.

I dont think Nintendo can win me over. The tablet controller doesnt interest me until I see a game truly improved by its use. The lack of online community and MP support is the biggest issue for me. I like that they seem to be getting a good amount of third party support to start this gen, but I have no faith in the online capabilities of Nintendo, and would not get a game like Aliens Colonial Marines for the WiiU cause I feel like the online features and gameplay will be wasted on the WiiU

Admittedly this E3 is rather lost by Nintendo by default, rather like how they won 2012 and 2010 by default (with new hardware).

A show-stopping 3rd party exclusive, along with a plate of the expected and unexpected 1st party games for both Wii U and 3DS will help Nintendo do quite respectably in light of the fact that most attention will otherwise go to Orbis and Durango.

Yeah, as far as enjoying E3 they don't need to do much for me. As far as "winning" E3 vs MS and Sony, what they would need to do something shocking and spectacular. If they slashed WiiU pricing by $50-$100, announced an awesome 1st party game - something amazing like a cutting edge 3D mario, a new, game-changing first party game like WiiSports was at the time, a shocking 3rd party exlclusive (eg Final Fantasy 15), and the arrival of a multiplat that people don't expect on WiiU like say GTA5. Throw in some wild hardware innovation like a new 3DS version that looked sleek and could function as a second WiiU gamepad - let's call it 3DSU - and Nintendo could walk away with all the buzz. I don't see all that happening, however.

well there are two games that wouldnt win me over but make me REALLY start considering a WiiU and one of them is pretty much impossible to get/make the games are
Star Fox
Banjoo Kazooi

They would win me over right there and then if they bought SEGA and all future SEGA games were exclusive to Nintendo. Adding to that the announcement of Sonic Adventure 3 and the unveil of the new HD Zelda.
On the 3DS front i could use a new zelda aswell and/or a new golden Sun and Pokemon in 3D. Not really in Nintendo's hands, but a new Grandia/suikoden/Chrono/Lunar would get me very excited for owning a 3DS.

This would be more to blow me away with excitment. But just a SEGA buy would do wonders for the Wii U in my opinion. It would add the more hardcore gamer to the system without risking the Nintendo brand.

Extreme disappointment from Sony and Microsoft + some decent 3rd party support

Games, games and more games.

Everything else is secondary.