150 characters seems like quite a lot on the face of it, but I've repeatedly exceeded this limit when explaining to a user why I'm rejecting their edit.
For instance, on this edit, I was hoping to write:
Please don't add hi/thanks/salutations; see meta.stackoverflow.com/q/2950/16386, but even then I'd have rejected this as "Too minor: suggested edits should be substantive improvements addressing multiple issues in the post."
Yes I could have shortened it to:
See meta.stackoverflow.com/q/2950/163863, but even then I'd have rejected it as too minor.
But we shouldn't be held back from teaching/helping users because of a basic character limit. (and this is just an example... there have been situations where I've had to omit useful information to the user just to go below 150 chars).