Was the Wii getting "next gen" games and included in the PC/PS3/360 multiplatform ecosystem? No
What's the principal pourpose of a videogame console? Playing games
Okay, so by that measure you can can see why some people didn't call the Wii a "next gen" system.
Are they wrong? No
Is that the only possible interpretation? No
Yes, if you consider "next gen" as a term that's synonymous with "cutting edge graphics."
There's just three definitions of the term today:
1. "Next gen" -> Any console in the 8th generation of video games (Wii U, 3DS, Vita, 720, PS4, PC)
2. "Next gen" -> Any console that can run cutting edge graphics technology in the 8th gen (likely the 720, PS4, PC)
3. "Next gen" -> The 7th generation ecosystem of games with cutting-edge graphics (PS3, 360, PC)
The Wii U is "next gen" because it's being released in a new generation, but it's not "next gen" because its graphics are roughly equivalent to the 7th gen of consoles.
HOWEVER, just because the Wii U isn't "next gen" in terms of graphics, doesn't mean it won't receive ports of "next gen" games that get released for the PS4 / 720. In fact, I think it's very likely that the Wii U will receive tons of ports, even if they're scaled down a bit.