You know, I've been meaning to make this thread for a long time, because I have the exact same question. The people primarily putting down Nintendo's network are people who don't even have Nintendo consoles, or who have a 3DS and not a Wii U.
Basically, you can sum up the zeitgeist like this:
-Nintendo's online sucks because it doesn't have achievements, cross-game chat, or cloud saves, even though it is completely free and plenty of people would take free online over a paywall online with those extra features.
-When most people talk about how Nintendo's network sucks, they're not talking about the actual ease of getting into an online game or the smoothness of the gameplay, they're talking about the missing features in the point above.
-People think Nintendo sucks because you can't legally bootleg your account on someone else's console and essentially pirate your games there. The truth is, most people aren't half as worried as their games being lost, stolen or broken as they are whoring out accounts across multiple consoles for free game access, a la Netflix.