The inventory
What was wrong: It's easy to forget just how much of Mass Effect 1 was spent in the inventory screen. Practically every enemy you killed had some sort of present for you, whether it was a weapon mod or full suit of armor. If you were the compulsive RPG type, every battle would send you back to the menu screen to see if you picked up some new gear worth equipping. Even if you had more patience than that, eventually it would come time to pare down your one-man arsenal before you went over the 150-item limit.
For Mass Effect 2, the inventory was cut entirely. You choose which of your 2 or 3 standard guns to take on a mission when you leave the ship, and your armors stay up in your captain's quarters where they belong. The result is less time navigating menus, and more time hanging out with Garrus.
What was right: We definitely agree that menus are less fun than gunfights. But the reason the whole idea of RPG loot is still around is that there's just something primally satisfying about killing a guy, taking his stuff and wearing it yourself. That makes RPG players sound like psychotic loons, but we dare you to deny it with a straight face.
There's a nice little moment of satisfaction when you trawl through your cache and find a rare armor that's way better than what you've got. That feeling of increasing power isn't totally missing from Mass Effect 2, since you do find new guns from time to time, but there's fewer spikes in your capabilities that make you feel like you've turned a corner. There's also no anticipation of saving up for that sweet HMWSG shotgun you've had your eye on.
ME2 also lost some customization when it ditched weapon mods. A clever player in ME1 could substantially change how some of their weapons worked, like putting explosive ammo in a sniper rifle, thereby making it a personal rocket launcher.
Additionally, it was kind of nice to be able to customize your squadmates. At the very least, you got to enjoy speculating how the Sirta Foundation ever marketed a line of pink armor to the military.
Above: "We've moved beyond your primitive gender stereotyping in the future, so it's a total coincidence that I'm wearing this"
Unlimited ammo
What was wrong: Actually, we don't think there was really anything wrong with this one.
Unlimited ammo was a neat concept that worked well in the Mass Effect universe. Each gun had a block of metal stored in a mass-reducing field, and "bullets" were pieces of metal shaved off of that block and accelerated. One block would supply thousands of rounds, and the only limiting factor was overheating. Future tech at its best.
What was right: The designers of Mass Effect 2 weren't quite happy with how the unlimited ammo scheme translated into gameplay. To make every shot feel important, they instituted a system of thermal clips, detachable heat sinks that act as ammo.
The in-game explanation for this is that thermal clips let soldiers fire faster by obviating the need to wait for weapon cooldowns. We can sort of buy that, but did they have to make the weapon USELESS without a clip? That definitely seems like a step backwards.
Even so, as a game mechanic, ammo isn't bad, exactly. We get why Bioware thought it would be better to make us conserve shots. It's more that, as sci-fi dorks, we're compelled to pick on inconsistencies in lore.
