Castle Crashers is a fantastic game, I've played it. Lots of fun.
It's hard to go by those numbers alone so I'll post the 3DS digital numbers:

There was a table with the actual numbers but I can't find it just now.

This is all I found for now, but keep in mind the attach ratio factor. If the 360 install base is huge, it's not completely apples to apples to compare DD sales on 360 to DD sales on WiiU.

Wii U eShop sales chart for the week ending on December 31 

1. Trine 2
2. Little Inferno
3. New Super Mario Bros. U
4. Nano Assault Neo
5. Chasing Aurora
6. Mighty Switch Force!
7. Sonic & All-Stars Racing Transformed
8. ZombiU
9. Scribblenauts Unlimited
10. Assassin’s Creed 3
11. Batman: Arkham City Armored Edition
12. Darksiders 2
13. Tekken Tag Tournament 2
14. Madden NFL 13
15. Ninja Gaiden 3: Razor’s Edge
16. Just Dance 4
17. Wheel of Fortune
18. Epic Mickey 2: The Power of Two
19. Nintendo Land
20. Jeopardy

I mean Xbox/PS3 owners have been exposed to digital distribution for the past 6 years or so. Naturally, they've become more "accepting" of buying digital goods.

How long do you suppose it takes to get accustomed to something so ubiquitous? Also, some people own other devices and even consoles from the competition where they already DL software, so I'm not sure how important a constraint this is.

When has Nintendo struggled at retail?

I see plenty of Nintendo games in the retail channel. All of the console manufacturers will benefit from DD but I don't see how Nintendo will have any advantage in this area considering they're actually quite far behind.

It isn't a perfect argument because there are too many variables, one of them being bolded there. However, Nintendo's biggest obstacle is it's ability to play the competition's game, they just straight out suck at it. MS/Sony aren't afraid to copy each other if it's gonna benefit them somehow. Nintendo on the hand has historically proven that they rather develop their own strategies than adopt an already successful one.

This question begs for an answer. What has Nintendo been doing since 2006? Was 6 years not enough for them to devise an online strategy for the Wii U? I wouldn't call what the Wii U has right now an online strategy, specially if Nintendo is actually serious about the Wii U.

This is of course my opinion so don't take it as fact. I have both a X360 and a Wii U and i haven't felt compelled to buy anything digital on my Wii U because 1. It's not tied to my account, 2. Not sure about the future of the system and 3. Depending on the game, the competition offering is often better specialy when it comes to online. I'd imagine that most multi-console owners feel the same way.

You do know that nintendo started VC back in 2006 right.. ?

And content are not locked to the consol but to the person who owns it.. meaning if the wii u break down (yes i know. that almost never happend) but.. then you could call nintendo and tranfer them to an other console... and i bet anything you will be able to tranfer them to next nintendo console to.. (or what ever nintendo comes up with)

Ont: One thing for sure.. Nintendo will earn more per consol than they did with wii.. much more.. and even if some wii u owners are kids (i think some kids have ps3 and 360 to.. but leave that) They are the most powerful consumer group.. (they dont have cards.. yes.. but they dont pay.. it's the parents job (or uncle or grandmother or.., other people like to spoil kids)

And think of it... What happends when mom and dad a going out to a party or so.. they first bye theire kids some nice nintendo games.. :)