Nintendo must have a good pace of quality software throughout its lifetime while also building/fixing bridges with third parties. If they prepare the necessary leg-up into the 9th generation, then the Wii U will be successful to me.

Success for Wii U is having it's hardware and software numbers grow

Success is measured later in life. Sony isn't successful yet with the PS4, neither is NIntendo or Microsoft. If Sony pulls of 70 million, and MS pulls off 60 million and Nintendo pulls 50 million, I would consider all three successful. If All consoles pass 10 million before the end of their life, that's a successful console generation in my opinion.

Then again, I measure success in how much fun they can deliver. Whatever console provides me enough fun is successful to me. I have games I hate and games I love.

The games are coming.. so that's not a question.. we have even (at this point of the gen) seen some new ip's we not expected.. In that case WiiU will for sure be a "succes"

It has to make money.. But that I consider part of..

1. Sell more than 50 mill lifetime
2 Be number 1 or 2 in 8 Gen
3 Keep Nintendo franchises fresh and loved
4 Out of the 20 best selling games this gen.. minimum 10 WiiU games

I think it will get all 4..

My fav answer, because I think there are a couple of factors that could risk losing a portion of their fans

The way I see it, Nintendo needs to generate some momentum this year with Hyrule Warriors, Bayonetta 2, and Captain Toad, leading into Super Smash Bros. Afterwards, they need to keep it going in 2015 with Yoshi's Woolly World, Zelda U, Mario Maker, Splatoon, Kirby and the Rainbow Curse, etc. If they do this, their sales will likely remain at the levels seen in June, with slight valleys and hills along the way. If this occurs, the Wii U will probably end up right around the XB1 in terms of sales, which I believe Nintendo would have to view as a success. The generation could be compared to the PS2, XB, GameCube generation, with PS dominating, and Microsoft edging out Nintendo.

As long as Sony and Microsoft are around, it's likely that Ninty is never going to get any big 3rd party support so for Nintendo to be succesfull both for their fans and themselves they need to keep trying convincing developers that their console has a market for everybody both casual and hardcore.

And if this doesn't work, like usual, we got dem Mario games!

i dont think going from 100mil to 25 mil is a sucess.50 at least

If it is able to make a profit in the end and have a user base that is very satisfied with what they got. Than it is a success for not destroying Nintendo's future. At least than Nintendo 7 will be in a good position. However as a success in general never.