IE9 has a 4095 selectors / stylesheet limit. ( you can read a little bit about that here ). Once that limit is hit, the rest of the styling rules are silently ignored. (thank you Microsoft employees for not even bothering with a error message to console)
I would a clean solution from the Meteor community on how to have Meteor serve the original css files (minimizing is o.k. but not concatenated) to IE9-.
We are doing a temporary hack of asking user to upgrade but that seems really lame since the solution is "simply" get Meteor to not concatenate the css files.