I don’t think in most cases the 20 character limit is an obstacle on the site. But there are times a smaller limit might indeed be a happy answer. If you like a post why is there a need to address it further with a y, n, 
when you can simply like the post. If you like part of a post but wish to add further to it most times I think an answer/response ends up requiring more than 20 characters and also adds to the discussion rather than detracts. I also feel that a small limit may in fact remove quality from responses when we should really be aiming for quality and showing why we might disagree or agree or add additional perspective to a topic (whether in agreement or disagreement).
Can an individual topic be easily set to allow less characters in a response or is it a site wide setting? If this can be done for those that require a simple single response then when a topic is created that choice can be made. Though in many cases why not just create a poll to ascertain the totals of agreement, disagreement or neutral responses to an post/topic. Any more wordy responses if the respondent wishes to add them, they can then post as a reply.
If we want to set a site wide lesser limit and keep more meaningful replies then perhaps not a setting of 1, 2, or 3 but perhaps 5 or 10 or even slightly higher (or some number between) so that if it is just a mark of agree or not that can be addressed by a like (and also perhaps generate a disagree choice as has been mentioned in other posts/topics) or leave no like of the post.
Could we perhaps instigate a similar system to flags eg agree, disagree, ignore (ie hide that post for the user)?
Re disagree buttons & disagree responses (I think there may be more & at least one in a closed area topic):
and as @phb posted in his response on this topic
