Theory: Keep is an old service that was scrapped during development
OK, so I have a theory about Google Keep. In a nutshell: it's a service that was in development a while back, but was scrapped sometime during development, probably quite late on. When it leaked a few days back, Google saw the positive reaction to it, and thought, hey, what harm can it do if we launch this? So they did.
Hear me out. Firstly, Keep's UI isn't completely up to standards with Google's Project Kennedy guidelines. It's obviously a Google product, but there are noticeable differences: for example, the grey header with the search bar and Google Keep on does not have the Google+ bit on and the settings cog icon. It's on the black bar instead, with is coincidentally the old black bar, not the new one that's used on Gmail and Drive itself. Play uses this old black bar too. I thought Google were phasing this out and transistioning to the newer black bar, but now I'm not so sure.
Secondly, the Android app is really nice. Now, from the leak late last week, we know that Keep had an Android app planned, but if the service is old, it wouldn't have looked like this; I think the UI and appearance on Keep will be Android 5.0's appearance. My conclusion: Keep's app has been given a lick of paint since the leak. Would only take an experienced Android developer a day or two to do.
Thirdly, there's no iOS app. This could be because Keep is old, and when it was in development, Google had no policy to build great looking iOS apps like Gmail and Chrome. Since Keep leaked, they wouldn't have had time to build a new iOS app in the 4 or 5 days since it leaked.
What do you guys think? Hopefully Keep will be updated at I/O, and Google wanted to get it out as fast as possible. It's really, really odd that it uses the much older legacy black bar; everything I've heard from Google is that it's being phased out.