Big improvement from it's predecessor, Army of Two.
Army of Two the 40th day has all the cool things that was in the first one, and huge improvement. Remember in in the first where you could 'Pimp' you gun? Now you can make it look like something more awesome, you can change it's colour.
You can now customize your weapon at any time (whilst you're not in battle, of course) which is a huge improvement than the restricted 'customize-every-level' method.
Another huge plus is that Rios and Salem now have a GPS, which can tag enemies and guide you to your destination. Once you tag an enemy, you can see their position, even behind walls. If you're playing a friend, this will be an awesome feature, but with the AI..well, the AI isn't that 'intelligent'.
Sometimes the AI messes up a little, for example, sometimes you have to shoot an enemy before they shoot back. This is just an occasional glitch though, so it doesn't really matter.
Surprisingly, the AI has been improved too, when you switch weapons, your partner will be smart enough to do the same! If you pretend to surrender to an enemy, your partner will play along until you shoot.
Another new feature is you can now 'subdue' enemies to save yourself from having to waste your bullets! You have to grab and subdue the commander at first, and his soldiers will stand down. If you grab a private (the lowest ranking soldier) for example, the commanding officer may just think "He's just a low rank, we can sacrifice him" and command his other squad members to either shoot you, or your partner.
You also have something called "extreme morality moments" in which you normally have a choice to do a good, or bad thing, which results in different consequences. This is a smart feature that the developers added, in which it could make players to do another playthrough, doing the opposite things.
The sound in the 40th Day are.. mindblowing, the voice acting is amazing, and the weapon sounds are awesome. For instance, if you customize the barrel of your weapon, it will make a different sound than the previous sound!
The multiplayer has improved, the games comes with four game modes (3 if you didn't pre order the game)
• Co-operative deathmatch
• Control, which is similar to CTF.
• Warzone
• and Extraction, which you get if you have pre ordered the game.
In overall, huge improvement from the prequel.