The temptation to just keep throwing more "stuff" into a strategy game can be very strong. More options! More maps! More items! It's better because it's more, you see? Firaxis scored a modest hit by revamping the old XCOM franchise with Enemy Unknown last year and, like many modest hits, publisher 2K was keen to keep revenue flowing with an expansion. Thankfully, the team at Firaxis didn't just toss the kitchen sink into the elegant balance between base-building strategy and turn-based tactics, but gave the player significant choices to make in Enemy Within. All of the fun new toys, however, do work a bit against the game's biggest flaw - after a certain point of advancement, the challenge and satisfaction of snuffing aliens dissipates quickly.
Enemy Within presents a new version of the alien invasion, one which nicely dodges the narrative problems regularly associated with prequels or sequels by styling itself a "standalone expansion". You are still the commander of a multi-cultural organization formed to thwart attacks from hostile aliens. The gameplay is split between commanding your XCOM squad on tactical missions and making strategic decisions back at your base in what equipment buy or research to conduct. In this new invasion, the aliens drop a resource called MELD that must be harvested on the tactical map. There are two canisters per map, and it's a nice diversion to move your squadmates to the little yellow phalluses to collect MELD instead of just murdering the aliens. You often have to balance whether the stuff is worth risking your soldier's lives, and that makes for some tense and risky maneuvers.
The MELD is attractive, because it's used to fuel the two new ways to make your team more powerful. MECs are, well, mechs controlled by human agents that can be upgraded with dastardly modules like flamethrowers, grenade launchers and a kinetic punch that always deals 12 damage. Once you have access to the parts - you have to research the aliens you kill or capture through interrogations and autopsies to adapt their technology - it's incredibly useful to use MECs as your shock troops. They can absorb a ton of attacks, and there's a handy upgrade which lets the MEC soldier heal every turn. The addition of the MEC class follows a completely different upgrade path than normal advancement and takes up a lot of MELD, but it was enjoyable to experiment with the new permutations, even if it felt a little overpowered having two or more MECs in your squad.
The other way to upgrade is to modify your soldier's genes using alien strains of DNA. These gene modifications are also discovered through alien autopsies, and they can be applied to any class, be it sniper, assault or support. Unfortunately, you can't do gene modifications on MEC soldiers, because that'd be cheating. Modding your soldiers takes them out of active duty for a long while - 3 days per mod, usually - and costs significant amounts of MELD - up to 35 MELD per mod - so you really have to pick and choose how you upgrade your squad's body parts. It may make the most sense to modify your sniper's eyes so that his aim improves the higher in elevation he is, and allow him to get to that height by modifying his legs to leap on top of buildings, but modding every new recruit just isn't feasible. Especially if they are just going to perish the first mission they are deployed because of an (un)lucky crit roll.