Yeah, I think having more options (like 5-item Likert) would be worse for exactly the reasons you mentioned.
It's a good point that you can't do a slider in email, I didn't think of. Unless email allows embedding an html form that you can submit? I've never seen anything more than static html in my email so I guess it must not be possible.
Anyway, thanks for your explanation, it makes sense. I think I'll make myself a 0-1 floating point favourites tracker and see how I like using it.
Yeah, forms in an email are tricky. Some clients actually do support them - which was a big surprise to me! http://www.campaignmonitor.com/resources/will-it-work/forms/
Thanks for the feedback, I appreciate it.
I don't want answering the question "how is your day going?" to take more than the 2 seconds it takes to move your mouse to the link.
The problem with the slider is that you have you interface with it. So the workflow would be: get an email, click a link, move a slider (compared to get an email, click one of 3 links). I know I'd want to avoid the extra step as a user, so that's why I opted for that approach.
There is also an option for the user to enter an anonymous comment after they have logged their mood - so if you want to express why today is bad or add note that it is an especially excellent day you can do that.
At a higher level view, I think the value in this particular app is not a scientifically rigorous computation of a teams mood, but rather as a conversation starter ("why has there been a few 'bads' this week? let's talk about it") and a general trend indicator ("morale is down since we moved to the new office, what's up with that?").