Argument
Keanu always underacts
Rebuttal
It's called subtlety. Most actors tend to overact in a way that looks good on screen but wouldn't be that realistic if witnessed in real life. Meanwhile, people are all different and all react differently. Maybe the majority of the population cry in the way portrayed by most actors, but there are also those who do so the way Keanu does, and so on for different expressions of emotions.
It's generally easier to overact than underact, and any one Hollywood actor regardless of talent is more likely to be guilty of the former.
Also, some guy brought it up on an online blog that while with most actors you watch their performance and are fully aware of watching them, Keanu usually plays it down to the extent that the viewer can imagine themselves in the place of his character and thus feel a part of the movie. Obviously this doesn't hold true in all his films, but for most of them there is a valid point there. Elsewhere, one of his co-stars mentioned how Keanu sometimes purposely underacts in order to make the rest of the cast stand out on their own merit without him taking up all the space.