Some thoughts I wrote on another forum:
I think the best word for me to describe it with is "pure". It is the "purest" gameplay we've ever had in Metal Gear since MGS2. Everything feels organic and smooth, there's a ton of freedom to allow you to play any way you want, and lots of bloat the series has accumulated over the years like camouflage (which is fantastic in an outdoor setting, but unnecessary in bases like Shadow Moses, Big Shell, and Camp Omega), instant access to an entire armory of weapons, and unnecessary items have been cut, and it seriously feels great. No longer do you have to go into your items menu to select "scope" every time you want to use the binoculars, or even the radar, AP Sensor, or directional microphone. The binoculars are infinitely more useful now due to the lack of a radar, as you can use it to "tag" enemies from afar, setting a small symbol above their heads that allows you to keep an eye on them, and you can also listen in on enemy conversations with the built in directional microphone. The game now has a little white indicator that fades in when you're in danger of being caught, as well as a slow-mo "reaction time" effect that let's you shoot out a guard before they call for backup. Both of these can be turned off, and doing so increases your score, but I did really appreciate the alert indicator in particular. It's much less distracting than the threat ring from MGS4.
The level design allows for so much freedom, it's seriously amazing. I've had three runs so far and none of them were alike at all. I can see what Kojima meant now when he said that "when one hundred people play a mission, you'll see one hundred different stories playing out". It feels like you're really in control.
Game's fucking good. I had more fun with that one mission than I did in all of MGS4 and Peace Walker.