I just had the need for this today. For now we can only use proposals, but I think at some point it might be useful to differentiate between formal proposals and temperature checks.
One might want to do a temperature check to see on how people feel about a proposition/an idea before a formal proposal is made.
The main difference with a proposal would be that a temperature check would have no outcome and you would not be expected to justify your choice. Also I think the 4 buttons would be replaced with a scale from -2 (very bad idea / I refuse to do it etc…) to +2 (sounds awesome / looking forward to it / count me in etc…). We could also use colours, emoticons etc…
It would be very similar to a poll tool, but I think it is important that the tool permits people to show dissent to what's being proposed (a classic poll tool doesn't necessary allow to do this).
I've added that feature to the Loomio functional overview (that will be migrated to Trello at some point)