I personally find the term 'gimmick' or novelty unhelpful. Everything is a gimmick in relation to a convention/tradition/accepted precedent. All consoles are gimmicks in comparison to monopoly or chess, all input modes are gimmicks in comparison other modes of communication, everything is a gimmick!
The term gimmick is quite flexible, and is used quite broadly. But ultimately if a feature isn't a benefit, and is only there to differentiate then its a gimmick.
You're right Nintendo haven't always bee gimmicky, but they have always been innovative. However they put themselves in the position where people think they just release gimmicky hardware.
Motion controls weren't a gimmick. People think they were because the industry has more or less abandoned them, but the reason for that wasn't consumer demand, it was because Playstation and Xbox were chasing hardcore gamers and Nintendo stupidly went down the gimmick WiiU route instead of just releasing WiiHD like they should have done. There's still an appetite for motion controls and I think we'll see plenty on the Switch.
Anyway similarly the hybrid aspect of the Switch isn't a gimmick because its a genuine evolution in gaming.
The gimmicks were 3D, that bloody WiiU tablet, the heartrate monitor and arguably dual screen gaming (I appreciate some people like this though). They were tolerated because of the good games on the system, but they didn't add anything.
Following the Wii, Nintendo seemed strung by their position as "innovative" and as a result they released a strong of products that innovated for the sake of it, without there being a need. That is where Nintendo's reputation as gimmicky has come from. 2006-2016. With the Switch I feel like they are over that now, but we'll see if they release another console with a screen that you can snap in two and stick one half on your fridge.