I wouldn't say Splatoon rekindled my love for Nintendo and gaming (Valve and From Software did that last year, actually), but Splatoon was the main reason I went from "meh, the We + You seems like an okay system" to "omg I need a Wii U ASAP". The thing with the Wii U is that while its library is definitely strong, with a good amount of quality games, most of those games don't appeal to me as they used to appeal back in my childhood. I love Mario and Donkey Kong, but I just don't have the same passion for platformers I used to (I still plan to play TF and 3D World, though ^^;
. Mario Kart is great, but after playing Sonic & All Stars Racing Transformed, going back to Mario Kart doesn't feel right (for a number of reasons), I could never get into Pikmin (the sound of Pikmin dying is too much for me to handle lol) and the list goes on. This is one of the reasons I was so glad the 3DS was getting Smash too, because I wouldn't need to bother with a Wii U only for Smash (and it so happens the 3DS version is extremely underwhelming in various aspects).
From my adolescence times onward, I have been much more into action/action adventure games (as well as on rail shooters like Kid Icarus, Panzer Dragoon and puzzle/shooters like Portal and Portal 2) and some RPGs here and there, so the Wii U library felt lackluster for my "modern tastes". Fortunately, at the last E3, Nintendo announced Splatoon and showed us more of Xenoblade Chronicles X, the sequel to the game I never finished, but acknowledge the quality. That, plus the fact Nintendo said they are planning two new games for the Metroid series (with Reggie wearing a Samus pin at PAX) sealed the deal for me. There is also Zelda U coming, which I know will be great and Shin Megami Tensei x Fire Emblem, which I am sure will never be released.
Anyways, I typed a lot, but what I mean is that Splatoon showed me the Wii U library wouldn't me composed mostly of "more of the same" games and that Nintendo still has the potential to create new, crazy and unique franchises. I am sure a lot of people felt more confident towards Nintendo after the past E3, too