Usually expansion packs are not as good as the original, they sometimes fix problems and glitches in the original but then they drag in there own problems, making the original feel watered down. XCOM: Enemy Within does none of the above. It fixes almost all the problems Enemy Unknown, brings in lots of new content, and it does all of this without bringing in its own problems. At first you won't notice that anything has changed because strip away the new content and its the same story as Enemy Unknown. You may notice the extra Second wave settings and other options, but at first everything seems the same. However, a couple of missions into the game and a lot of the new content makes itself known. For a start there are 40 new maps! In Enemy Unknown, shooting down a UFO always ended up with you having to send your soldiers into the middle of nowhere, because luckily UFOs always went down in sparsely populated areas. In Enemy Within UFOs will sometimes go down in the middle of cities, requiring you to search the inside of a half destroyed skyscraper looking for the UFO crash sight.
There are also alot more preplanned missions where the council asks you to perform special tasks for them, and some of these are stories in them selves. And it is while you are on one of these special missions when you discover MELD.
MELD is a strange alien substance that combines mechanic with organic blah blah blah science science science, long story short it allows you to research genetic and cybernetic enhancements for your XCOM soldiers. You must build a Genetics or Cybernetics lab before you can perform these projects. Genetics allows you to use the MELD to upgrade a soldiers internal Organs or their skin. For example Super reactive pupils upgrades a soldiers accuracy and "second heart" makes a soldier harder to kill. Cybernetics involves removing all of a soldiers limbs and replacing them with machine hands and legs, this allows them to take control of MECs. Genetic soldiers are basically improved XCOM soldiers with new abilities, such as the ability to turn invisible. MECs are as powerful as they are awesome but to maintain balance in the game they can't use cover, so strategic positioning is the key to their survival. You find MELD in abduction missions and in UFO missions, MELD is kept in canisters rigged to self destruct and you collect them the same way you disarm bombs. Recovering MELD can sometimes be an infuriating task that will have you pulling out your hair. To help you recover MELD your soldiers will sometimes hear it twinkling, it then shows a line pulse across the screen in the general direction of the MELD canister, this is similar to when your troops hear aliens before you can see them. However, when it plays the sound trace when your soldier hears something, the whole screen pans to the soldier while they say "what was that noise?" giving you a good long while to see the noise trace. When a MELD trace pulses across the screen your soldiers keep their mouths shut and the screen doesn't pan across to it, this means that you have to be in the right place at the right time to see it. As if that's not problematic enough, sometimes the game just doesn't give you enough time to grab the MELD. On one mission a MELD canister is around four turns from the starting location, you have around five turns to go and recover it, but on your way towards it you encounter a gang of Muton, wasting two turns while dealing with them. When you do finally locate the MELD you only have one turn to reach it and can't get a soldier to it in time. Thankfully these problems are rare, with two MELD canisters per mission you usually get at least one of them. However, sometimes you will ask yourself is it worth the risk dashing for a MELD canister surrounded by Floaters or if you should just deliberately detonate the canister to blow the Floaters even higher into the sky
As you use the MELD to improve your troops you will find that you are not the only one using the MELD to your advantage. There are two new alien types, Seekers and Mectoids. Mectoids are Sectoids using MECS, they can be quite difficult, reducing cover to crumbs and leaving your soldiers as no more than red stains, but they are fun to kill. Give one of your MECs a melee weapon and order them to use it against a Mectoid and the results are fast, deadly and awesome. Cool camera angles and a scripted fight scene show your MEC battering the life out of the Mectoid. Seekers are weird looking squid robots. Unfortunately, while frightening and dangerous looking at first glance, they turn out to be as easy as Sectoids. When they see you they immediately turn invisible, a few turns later they reappear next to one of your soldiers and wrap their metal tentacles around his or her neck and starts to crush the life out of them. The idea seems to be that you have a certain amount of time to help them before they die, and I must say its a good idea but a flawed one, because your surrounding soldiers will have no trouble hitting the Seeker. This is made worse because Seekers don't have that much health and by the time your soldiers are using beam weapons they will be able to kill them in one hit. In the end Seekers are only good for target practice and cheap promotions.
Another new threat that Enemy Within creates is EXALT, and they are one of the great highlights of this game. Exalt differ from the other enemies of Enemy within in one way, they're Human. They are a group of Human traitors and perform anti XCOM operations, they might hack research data or steal your hard earned cash, and the longer they go without you taking action against them the more damage they do. Exalt cells operate in different countries, sometimes they reveal themselves to you when they hack into XCOM but sometimes you must look for them by scanning each country in the world. When you discover which country they are operating from you must send a single soldier under cover to prepare for a data recovery mission. After around three days you must send your troops in to protect the undercover soldier while they finish recovering the data, and at the same time destroying the EXALT cell. After each successful EXALT mission you narrow down which country their main base is in. Battling EXALT is very entertaining and the missions are well designed.
XCOM Enemy Within doesn't just add new content, it also improves some of the big bugs and glitches from XCOM Enemy Unknown. The cinematic camera when a soldier kills an alien, in Enemy Unknown was always glitching and clipping through walls, Enemy Within has fixed the insane camera. Now it only starts the cinematic animation if the camera has enough room to do so. It also improves the death animation for the aliens, in Enemy Unknown the aliens wouldn't so much as twitch when one of your soldiers hit them with bullets, not until it had finished the shooting animation. However when an alien is shot in Enemy Within it plays the animation as soon as a bullet touches it, then after that it plays the death animation. This makes the combat seem more believable and less arcade like. The Ragdoll is also improved, all in all the death animations are more believable than they ever where in Enemy Unknown.
What's really impressive is that usually expansion packs come with a unique glitch or two that wasn't in the original, but XCOM Enemy Within is practically flawless. If I was forced to say anything negative besides the Seekers and the problems with collecting MELD I would say that the load times are slightly longer than before, and that it sometimes has trouble loading each individual soldier while preparing for a mission. But that's only a small problem in a expansion pack that fixes way more problems that the two or three it creates. In short Enemy Within leaves XCOM in better shape than before it arrived.