Bart summed it up nicely. The ACR and the SCAR are, currently, priced fairly closely together, but the SCAR has more features. The ACR promises that it will eventually have additional features, but at what price?
As far as the quick-change barrel and caliber-converstion argument goes, I certainly understand that the ACR cannot be expected to be supplied with a full field of OEM and aftermarket support right away. However, Bushmaster seems to expect that people will pay a premium for the base rifle with zero ACR-specific accessories available, and I suspect that they may be disappointed. We'll see. Compare/contrast this with Robarm and the XCR, which was released with more features than the current ACR, but with a lot of future potential still in development, and guess what? They priced it accordingly. Buy the base rifle now, buy the add-ons later. That's more reasonable than "pay inflated SCAR prices now but get less, and maybe you'll be able to pay us more later to get the full package. Or, maybe you won't. Do you feel lucky?"
The whole quick-change barrel, caliber-conversion capability of the other next-generation rifles (ACR, SCAR, XCR) has not been fully exploited. The XCR is probably the furthest along, and even it is not as developed as it could be. And you know what? As eagerly as I was anticipating the XCR, ACR and SCAR...I don't own one yet. That's not a coincidence. I see no reason to pay through the nose to be an early-adopter/beta-tester when it is not at all clear that any of the companies in question intend to follow through with their sales pitches.
If the ACR functions properly, they come out with the additional features, and the price becomes reasonable, I'll buy one. Until then, it's not worth it.
Mike