I think that a major difference in how many of us view the separate acts is the manner in which they were planned and executed. McVeigh spent months planning the attack, getting the materials, and finally committing the act. McVeigh's chosen method (bombing a building) mimics the conventional mode and weaponry of a terrorist attack. The site of the bombing (an office building filled with civilians) mimics the type of place we think of when we think about the typical terrorist attack.
Hasan’s attack has the earmarks of a typical shooting spree: man with gun shoots and kills several of his co-workers. It may have taken him months to plan, or it could have been a spur of the moment type of act. It doesn’t matter which it was, but you would have to admit that the act itself is vastly different than McVeigh’s in the way it was executed, and in the way that it fits into our views (perhaps outdated) on the way terrorist attacks are committed. I am hesitant to say it was a planned terrorist attack, but as more evidence has come to light I am willing to concede that those who are calling it are terrorist attack have a point, something I posted in the other thread a couple of days ago: