Since the character limit of Chatter/feed comments is fixed & relatively low (for a number of scenarios), it would be helful to have a character counter like you see on most web text boxes. Ideally it would show number of character out of the max for that org/instance. Color coding the counter would be a nice touch as well (green when under limit, red when over).
Here's an example:
There are 2 other ideas asking for this as an addition to increasing character limits but I feel this deserves it's own voting due to it's usefulness. Here are the other ideas: