Short answer is a very crisp dot."What about the RMR makes it appealing for carbine use?"
I am a FIRM believer in Aimpoints for LE Patrol rifles and have been well served by Aimpoints for many years. However, since my accident in 2009, the Aimpoint dots now appear as misshapen blobs to my damaged vision, especially when using a 4x magnifier. In contrast the 3.5 MOA RMR06 dot is crisp. At 100 yds, it perfectly fits into the center of an SR1 target. The RMR06 mated to an RM34 mount or similar mounts by LaRue or ADM makes for a very low profile optic.
This guy thinks they RMR's are OK on carbines:
Note that the photo of Pat above was from a 2 day class conducted in the rain as you can see from the puddle of water pooled in the background; the RMR's in the class exhibited no issues.
And here is the one I use: