My own experience, though limited, also shows that intensity can be perceived as violence. I would agree with Kevin Leavitt that intent is the differentiating factor between the two. However, one's intent is a category known to oneself, not necessarily to one's partner. Provided there is enough trust between the two, no conflict should occur. On the other hand, a lack of honest communication (not necessarily verbal) or even bad experience from previous training sessions might draw people to get uncomfortable on the tatami.
It seems though, that this question arises more frequently among those aikido-students that only just began training. Or is that just my experience?