In a word: granularity.
The Wii U is coming out on Sunday. Nintendo has detailed the process for transferring your Wii game save data, and downloaded games from the Wii to the Wii U.
That's fine, and I get that people want to do that. But from what people have reported, it seems like its an all or nothing transfer, and it's a one time only event.
Here's where I have a problem. I want finer control over what gets transferred and what doesn't. Specifically in my case I want to leave the downloaded Wii Shop games (Virtual Console and WiiWare) on the Wii. But I want to transfer my game saves and Mii's to the new system.
Here's the rationale. There's no advantage to having my VC games on the Wii U. They will look just the same on my HD TV. My plan is to bump the Wii down to my old CRT TV and play virtual console games there. Where they will actually probably look half decent in standard definition.
On the other hand, I still want to keep playing some Wii games on the Wii U, like Super Smash Bros Brawl or Monster Hunter Tri, without losing my game save data and being require to start from the beginning again.
So I'm going to wait, in the hopes that Nintendo releases a transfer process that is a little more fine tuned, than the current all or nothing proposition.
Who knows, maybe they'll surprise us on launch day with a tweaked transfer process? That would be a pleasant surprise!