So, is it the "crab" in the name?
Is is the way that it grows?
Its been referred to in other posts as an "infestation", so is there something negative or dangerous about it?
Or is it that it is invasive?
(In other words, due to it hardiness, crabgrass is dangerous competition to a cultured lawn?)

It was always in the lawns of the military bases that I grew up on. The three-stemmed seed tops always reminded me of helicoptor blades. And what with my father being a helicoptor pilot, there has always been a loving, familiar association with it.
Other than it not being as pretty or its invasiveness, what's the problem with crabgrass? (And by the way, I'm not defending crabgrass, either. I'm looking for a more efficient lawn alternative as we speak. That is, if there's such a thing.)

Silvaworks,
Crab grass is considered a weed to many homeowners, parks, golf courses, and other facilities with high maintenance grasses because as you mentioned it is unsightly with its seed heads, doesn't look like other grasses (maybe St. Augustine), and can choke out other grasses in harsh environments.

Crab grass is a grass. But its growth habit (bunched) is not suitable for growing a nice lawn. If it ever fills in, it would take a very long time.

Crabgrass can be a real headache to many turf managers as it can be hard to control once it has emerged and becomes mature. Even multiple applications of Round-up must be used to kill mature Crabgrass.

Oh, and it cost turf managers a lot of money to control it so that is why most people dislike it .