Well I've always been into Smash games and Nintendo games in general so it's just something I thoroughly enjoy.
For someone coming in without that same feeling it could definitely still be a great game for you.
The different modes help provide fun for everyone. Once you have the basic controls down pat you can play your 4 player - 8 player modes with items on/off depending on what kind of madness you want. When you start to get better that's when a lot of people tend to enjoy playing with no items and just have certain number of stock/lives.
The game is just absolute chaos and so much fun. I think your wife would enjoy it as well, as long as she is somewhat into video games. I never tire of playing online having 1 on 1's but it's also great to play local multiplayer and just have some crazy matches.
I know there was a demo on the 3DS, is there a Wii U demo you could look at getting first?
