Not as many bullets as a 9mm
Not as fast as a 10mm
Not as fat and heavy as a .45
That's a lot of things it's 'not'. I'm still not sure what it 'is.' (Arguably a compromise in speed power and capacity made popular by police forces and the power of Glock and SW marketing...but if you already have a 9mm and a .45 why would you need one?)
Come now, that's not even being honest. Just say "I only like .45ACP" and be done with it. I suppose the "40 haters" also dislike .357 sig for the same reasons ("it's not .45" or "it's new and has no mystique"), despite its ballistic similarity to .357 mag.
How about:
More bullets than .45
Less kick than 10mm
Larger than 9mm
Personally, I like 40S&W for a number of reasons:
1) It's not as expensive as .45ACP
2) the 3 above stated reasons
3) a compromise between the 'snap' of 9mm and the 'push' of .45, allowing for a less jarring experience while shooting than 9mm, and a faster recoil recovery than .45
4) There aren't a whole lot of carryable guns available in .45 that aren't 1911s or entirely too fat. Thus guns like the CZ75 or BHP in .40 come into favor.
Really, if you're used to .45 or 9mm, you'll end up limp-wristing .40. A .40 USP was my first handgun, and I've learned how to shoot it properly and make quick recovery shots. I like the 'twisty snap', and like many of you feel about the slower push of .45, you learn to let your body work "with" the recoil and not against it, working out a subconscious response to the recoil.