Potentially even worse, since it's so insidious, is that the fear of getting put on "the list" can have a chilling effect on the perfectly legitimate activities of loyal, law-abiding citizens.
Suppose you're a person of color or muslim, who happens to be interested in learning about the enemy (e.g. Al Qaeda). One obvious way would be to look at their forums online. But wouldn't you be at least a little bit afraid that someone's watching for people browsing to those sites, discovering your ethnicity and background, and slapping you onto the list?
From the other side, why wouldn't the watcher put you on the list, just in case? There are no negative consequences for the individual doing so (because no oversight/warrant procedure), and what if he/she "missed the next terrorist" (i.e. maybe you, as far as they know)? It's a no-brainer and could even be rationalized by otherwise well-meaning people as being patriotic. Imagine the poster on their cubicle wall of the burning WTC towers, with "NEVER AGAIN" underneath.
This warrantless, unsupervised paroxysm of fear has got to stop. We are Americans. We do not live in fear. We do not sacrifice our freedoms for security. We are willing to accept that some of us may have to die because we exercise those freedoms, and we know that the answer to "What if there's another 9/11?" is "Then we will survive and triumph through that, too."