I was doing the Wii to Wii U transfer and it was taking awhile and I fell asleep, woke up and my Wii U was still on, does that harm it at all?

Realistically? No, but I wouldn't make a habit of it. At the very least it's a waste of electricity.

It's probably not a good idea. That overheated my Wii, and now it makes noise. A few times though probably wouldn't hurt it much, just don't leave it on 24/7.

I wouldn't have thought so, but considering the console is only a day old, you'll never know for sure. I honesty wouldn't worry about it.

I'd say it'll be just fine. It'll just use a bit of electricity. Won't overheat or anything, unless it's like in a plastic bag or something

Hmm, I leave my Wii on all the time because of Netflix, was never worried about it doing that since it's low power and not that strong. Thanks for the feedback everyone.

Yeah. I now completely power down my Wii(not sleep mode). But, for instance, when there is no sound coming out of the speakers(like when launching a game) it makes a whirring sound(the fan, I'm sure) and a beeping sound for some reason. It doesn't hurt my Wii, and I'm not sure if it hurt the Wii U(no one's had it that long)but it's a good idea to at least have it in sleep mode every night.

you should probably turn it off if you dont use it or nothing is being downloaded. system should handle it but why "waste" electricity?

I leave my Wii on all the time, a bit of a waste of electricity, but it shouldn't hurt anything.

The Wii U never turns off, its always on, even when your sleeping, the Wii U is watching you.
_{The Wii U must have some awful games, if all he does is make Wii U thread and not play his Wii U}

The Wii U isn't a sony product, but when me and my brother were kids, we left our PS2 on ALL DAY EVERY DAY. It eventually died because we didn't turn it off so we needed a repair. So my advice is: Don't leave your Wii U on, because it will raise your electric bill like hell and it may be in risk of dying.

Well, it does have a feature that if nothing is happening for an hour, it shuts itself off automatically (which can be disabled if you so wish).

Its like that for every console. Its ok for me on accident/downloading something. Don't let it become a habit.

In the short term? Probably fine. Heck, I still remember the days of the Gamecube and Smash Bros Melee where the common 'strategy' to unlock Mewtwo was to leave the game running with four controllers plugged in overnight due to the whole 'play 20 hours' of matches requirement'. Over the long run? Not so much, the electricity usage would be hell.

No, as it states on page 34, section 4, line 1 your Wii U HandBook, it will burst into flames if you do that

Its better not to leave any game on, but with that being said if Nintendo want the Wii U to be a tv controller/interface they should have the cooling being good enough to not overheat like a launch xbox 360 red ring of death.