If there is anything that has really been letting me down lately, especially with Nintendo, it is when games reveal most, if not all, of the content to be in the game without leaving any surprises. I am sure that most of this comes from the reasoning that people want to know what they are getting with their purchases, but in some cases I do not think that leaving surprises would have affected sales much at all.
Significant examples include:
Pokemon X&Y revealing many of the new pokemon and mega evolutions before release. After seeing so many, I had figured that there were surely a lot more left unrevealed. As a result I felt more disappointed than I would have been since I had expectations built. Maybe it is my own fault, but I was hoping for an experience more like Gold&Silver and Ruby&Sapphire where I did not know what was ahead.
Mario Kart 8 revealing all tracks, characters, and items. More or less the same deal, though I suppose it bothered me a little bit less.
Super Smash Bros. for Wii U/3DS revealing new characters, stages, and items. Here is where people wanting to know what they are getting with their purchase is most significant. Though most of the let-down came from the unlockables mostly being content from previous Smash Bros. games. Which is strange really, I figure that returning content is more likely to affect someone's purchase. I know someone who completely avoided the game simply because the Ice Climbers did not return. Right now I still have more songs to unlock in Smash Wii U that I lack motivation to go through with because I know it will most likely be nothing new or another Jungle Japes. While many of the old songs are great, I believe that I already have my favorites anyway.
This probably is not much of a concern for most people, but for me secrets and surprises are what really help me enjoy a game. Given the origins of Project Ukulele, it seems like this should not be a problem but I still wanted to get my thoughts out there, even though my thoughts are usually stupid and not worth expressing. A simple solution for me would be to simply avoid being spoiled, but that failed for both X&Y and Smash4 because they were being talked about everywhere anyway. In addition to that, there are some things that I would like to know, like release date(s), platform(s), special editions, the favorite color of each team member, etc.
I think levels, moves, and characters should each be kept relatively secret. Obviously there may be a few to be shown for various reasons, but overall I would like to be surprised as I progress through the game. I think teasing some things is fine too. I am sure many people, myself included actually, would really like to see what is in the game. However, I want to build up that excitement up until release and maintain it all the way through the game. I think something like a beta or early access release would also go against this a bit as well.
There are probably at least a few other people who have similar feelings, but if it is not the majority then it is not necessary too worry much about it. So ultimately, how many people wish to keep most of the content in the game as a secret until they play it?