One important note is this will make Macs an even more unattractive target for cybercriminals.
* Gatekeeper is enabled by DEFAULT, most users won't and don't know how to change default settings
* Power-users who disable Gatekeeper are probably already cognizant about malware and won't likely be tricked into downloading it
Ergo, this makes the potential target area for malware attacks on Mac even smaller than it is now. It simply won't be worth the investment for cybercriminals to go after Macs.
Gatekeeper is a primary deterrent against malware like MacDefender. Remember how the cybercriminals would release a new variant immediately after Apple added the last one to the Xprotect blacklist? Gatekeeper nixes that strategy. Gatekeeper + Xprotect makes Macs damn near invulnerable to malware when you think about it, and this even takes the "dumb user" factor into account.
Smart move, Apple.