These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Anticlimax Boss: The Alien Brain at the tail end of UFO Defense. See the page for examples of just how pitiful the final fight can get.
The Cthulhu expy at the of Terror From the Deep. It's an Eldritch Abomination, and it can't hurt you. It doesn't even have strong enemies protecting it like the Alien Brain did.
Author's Saving Throw: After the not-so positive reaction to the FPS reboot, finally announcing and focusing their marketing much more heavily on a strategy game that's faithful to the original style by Fireaxis Games (with many of the staff that worked on the originals at Microprose in the dev team) seems to be this. It helps that the FPS was delayed into 2013, and it was Retconned to be set in the past of the XCOM universe, derailing its status as a Continuity Reboot.
Civilization II had a track named X-COM (later renamed to They're Here), which is basically a remix of menu theme.
Most of the remakes themes, in particular the combat music.
Broken Base: 2K Marin's XCOM, either you think it has a shot at being worthy of the name X-COM, or you don't. There's no middle ground. Notably slanted in the direction of the "don'ts".
Contested Sequel: While the first game regularly tops (or is at least present in) various "best game" lists, opinions on the sequels are another matter entirely:
Terror from the Deep: Challenging upgrade to the original game with an awesome Lovecraft-esque atmosphere and a few forgivable bugs, or samey, buggy Mission Pack Sequel?
Apocalypse: A superior update to the formula with an excellent real-time combat mechanic, or an unfinished wreck with terrible graphics?
Interceptor: Fun space-sim-lite that kept most of the series' intrinsic mechanics, or a rather terrible attempt to cross genres?
Enforcer: ... Actually, most people agree this one was terrible, though you will occasionally see people mentioning that it's not so bad if you A) Can forget it's supposed to be part of the X-Com series and B) Are in the mood for an arcade-ish shooter.
XCOM: While most of the longtime fans are either shaking their heads at the decision to make it an FPS or are at the very least mildly baffled by the direction the developers are taking the game, fans and non-fans alike seem to be split on the enemies shown so far: Either you find looming, faceless geometry and mobile darkness frightening, or you just aren't that freaked by flying Rubik's Cubes and garage floor scrapings.
As for the commercial and freeware X-Com clones... bring an asbestos suit.
For Enemy Unknown, you either think it's been dumbed down to the point of being unrecognizable as X-COM, or modernized in an effective and enjoyable way that can be appreciated by new audiences and old fans alike.
Chryssalids. They have as many as 80 or more Action Points/Time Units on the hardest difficulty (able to go from off-screen to melee attacking in one turn), and as little as 60 on the lowest. To move one tile takes 4, or 6 if diagonal. Thus, they can blitz your troops from alleyways and corners. On top of that, if your surprised agents kill the Zombies created by a Chryssalid with reaction fire, out pops a new Chryssalid.... with FULL TIME UNITS and ready to make more Zombies out of your men. Chain reactions of frightened soldiers killing zombies with Reaction Fire before getting zombified by the freshly hatched Chyrssalids are the number one cause of Total Party Kills when battling against Snakemen.
Ethereals (champions of psi-spamming) deserve special mention as frustrating and unfair enemies.
Later in the game, this is reversed and a strong squad is virtually invincible, not in the least because you are the one making with the psi-spamming at that point.
Terror From the Deep, however, brings them back in force:
Tentaculats take it to a whole new level with their ability to fly when underwater. Combine this with the fact they love to hide in little nooks and crannies (which exist in alien bases for this sole purpose, it seems), one Tentaculat can decimate an ill-prepared attack force single-handedly. One particular problem with these monsters is that they only show up on alien base attacks, which means the first time you go to an alien base, you're in for a surprise. And lest you think that you can ignore the alien base and therefore the tentaculats, leaving a base active will decrease your funding score and increase the chance of nearby countries signing alien pacts. You're screwed either way.
Bio-Drones: Don't you just love enemies that upon being shot, immediately spin around and return fire with 100% accuracy?
The Lobstermen, who are basically supercharged Mutons with ridiculous resistance to almost all forms of weapons. Only the Drill are guaranteed to kill them fast.
Apocalypse is no slouch here, either.
Poppers are tiny annoying self-propelled bombs that close the distance to your troops in an unfairly short amount of time. If you're playing in turn-based mode, pray harder for reaction fire as they approach.
Psimorphs take forever to find, an eternity to kill and all this while easily wreaking havoc upon the fragile minds of your agents. Oh, and they can also fly.
Do you remember the tanks from the previous games? This time the aliens have them. Capturing a live Megaspawn and not losing half your force in the process is quite possibly the single most challenging task in the game.
Anthropods and Skeletoids are comparatively benign compared to the above examples, until they start carrying advanced equipment like cloaking fields and Entropy guns around just to make sure that you never catch a break.
Last but not least, there are Brainsuckers, coming to turn your agents against you and there's not a bloody thing you can do about it (unless you're playing in real-time mode, in which case they are much less of a threat). Unlike Chryssalids, they can also jump.
Good Bad Bugs: Most every release and patch of UFO Defense has introduced or fixed a couple of these. Whether they are to be exploited or patched depends on how Nintendo Hard you want your X-COM experience to be.
A fun exploit involves hitting certain sides of the UFO with explosives; due to imperfect wall structure, explosions can "leak" through certain parts of the UFO hull. Roast the aliens in their (UFO) shell...
You can stand on a bale of hay to see and shoot through the ceiling at the aliens on the floor above. Especially since that they're usually camping at the top of the stairs to ambush you when you come up.
The way mind-control works, you control 4-tile units one tile at a time. Aliens will automatically attack mind-controlled units within their line of sight when they move. So the best way to deal with a Cyberdisc or Sectopod is to mind-control one tile. When it moves, it will attack itself.
Because of a misaimed pointer, all terrain in mountain engagements has the blast resistance of a tree stump instead of their original resistance. In short: Everything-Is-Smashable Area . Even the outer UFO walls can be breached with regular explosives! But don't sneeze too hard while still inside the Skyranger...
An overflow error in the storage inventory data can cause the otherwise inaccessible Alien Reproduction facilities appear in your storage in the first game.
Internet Backdraft: The reaction to the FPS reboot was vitriolic enough for 2K Marin and Take-Two to add more strategic and tactical elements to the game in order to placate angry longtime fans. When that only made things worse they announced a Firaxis version which was already in development well before the FPS version was announced and now it gets more attention from the brass as a more apparent sell-able product when it got a far more positive reaction, despite the the fact initial announcement could almost be considered an after thought.
"Where are the Sectoids?" Depending on where you are in the series, they might be: on Earth and Mars, lurking at the bottom of the ocean in the form of surgically altered genetic throwbacks, working on a doomsday device in outer space while being harangued by Ethereals, trapped in the Alien Dimension awaiting processing into food, on Earth once more and looking very polygonal, sitting in an unemployment centre somewhere in Canberra or Novato, or up to their old tricks once more.
The 2012 TBS reboot has Lead Designer Jake Solomon's "That's XCom, baby!" whenever something unfortunate happens.
Stop Helping Me!: The scientist in Enforcer will leave you with a pathological need to kill him. Sadly, even at the beginning when you can actually see and shoot him, it has no effect. The Aliens get to him on Medium and Hard difficulties, in the end, but this lacks some satisfaction.
Both Enforcer and the upcoming reboot have taken a lot of flack for ripping out the original trilogy's turn based strategy roots.
With the reboot now regulated into a spinoff (along with fan complaints nullified by Enemy Unknown 2012), fans of THAT game are complaining that it has shifted from a investigative FPS to a Mass Effect styled 3rd Person Shooter.