What I like:
Arma's computer-controlled infantry has always been slightly schizophrenic. In Arma 2, they operate as dumb targets that occasionally demonstrate insane marksmanship, making them a simultaneously predictable and erratic enemy. From what I've observed in the alpha, Arma 3's AI hasn't fundamentally changed, but it has gotten better about moving more aggressively toward you from long and medium range. AI also lean and shoot around corners with greater frequency, and when they do spot you, they're good at remembering your position and firing on you quickly if you move from the left side of a rock to the right side, for example.
The new ragdoll death animations, which finally do away with the pre-baked death tumbles of Arma 2, are probably what add the most the fun to engaging infantry.
What I don't like:
Old behaviors seem to die hard for Arma's AI. Though there's more variation in how enemies respond under fire, the typical AI reaction is the same as it was in Arma 2. When I shoot at a squad a couple hundred meters away, they perform this dance: drop prone, get up, run a few meters, stand around, drop prone again, and eventually return fire. I really wish enemies simply sprinted for cover when you shot at them.
This aloofness also sometimes extends to vehicles. Friendly vehicle turrets seemed hesitant to engage ground targets in situations where there were plenty of targets to pick from, firing in polite, single shots when they should've been going full-auto.
What I like:
Arguably any change would've improved Arma 2's current inventory system. But what's instantly noticeable is how responsive and easier to operate Arma 3's gear menu is. Items no longer take a quarter-second to move or equip, and the addition of right-click and click-drag functionality for moving stuff around makes re-equipping much faster.
The main improvement, though, is the modularization of infantry equipment in Arma 3. Watch Dslyecxi's video above for a great explanation about what this system will mean for Arma 3.
What I don't like:
When you're looting a dead soldier, the “View All” selection doesn't display items stored inside enemy backpacks and harnesses, so nesting is still a minor problem. Relatedly, in order to steal a weapon attachment off an enemy weapon, I have to equip their weapon, drop the desired attachment on the ground, drop the enemy weapon, pick up my original weapon, and then equip the enemy's attachment.
Encumbrance is also only currently represented as capacity bars that fill up as you place more items on your soldier or in your ruck, and I'd like to see more feedback about its actual effect on stamina and weapon handling.
For anyone that enjoyed Arma 2, Arma 3's alpha is absolutely worth the $33/£20/€25 Bohemia is asking, the lowest tier of the Arma 3 pre-order scheme. It's low on content, but the most important thing—the sandbox—is accessible, and should be populated further in the coming month as enthusiastic community members fill the void. If this would be your first trip to Armaland, I'd recommend watching more video content before putting down cash, or grabbing an Arma 3 Lite key from a friend who's pre-ordered.