When the original Half-Life engine was released, many budding game modders were overjoyed with the possibilities opened to them. It's no surprise that many of the most successful game franchises arose in this time period, when modding was relatively easy on a modern graphics engine and developers were smart enough to recognise titles with potential. The most famous mods to materialize in this period were Counter-Strike and Team Fortress, two of Valve's most successful franchises.
Another lesser-known mod that was hugely popular was Natural Selection (NS). There were two things that made this game stand out from the crowd: this was a FPS/RTS hybrid that functioned well (who knows how many unsuccessful hybrids had existed to that point), and the universe was set in an Alien-esque setting. One player in each match takes a commander position and sees the game from a top-down perspective, and is responsible for setting objectives and creating buildings. The rest of the players try and do the (hopefully competent) commander's bidding. It's a complete RTS with supplementary bases necessary to tier up and resources that are shared between the commander and troops. The marines have progressively more powerful guns including heavy armour and flame-throwers, but are generally slow and vulnerable. The aliens are very quick, can climb walls, cielings and vents (in itself very fun) to ambush marines. They also have Fades that can teleport short distances and Onos which are huge tanks.
At its heart, this game was great because the pace was dead on. From the moment the game starts, there is never a dull moment, and two evenly-matched teams can have very long games that continually ramp up in excitement as both teams progressively tier up to more devastating upgrades. The mood was also excellent, with cramped and dark caverns or spaceships that would slowly fill with alien infestation. For any Alien vs Predator fan out there, Natural Selection was the game that did the franchise better.
Unfortunately, the NS developers were unable to get Valve's backing as they were already supporting other games, and interest for the game progressively lessened. Developers announced that Natural Selection 2 was underway as early as 2010, and that they would be using their own proprietary graphics engine. Most gamers thought this was a death-knell for the franchise (there's a reason that developers use off-the-shelf graphics engines), and it did take nearly three years to come to fruition. The result surprised everyone, because it simply looks gorgeous. Lighting, which plays a crucial role in setting the mood, has improved substantially. Nothing is more troubling than having to use flashlights to spot aliens when all that can be heard is their footsteps. It means that taking out power grids in order to remove lighting in areas becomes a priority for most alien teams. Performance is also great because the game is more CPU-intensive, so having a high-end graphics card isn't a necessity.
Aside from the engine, there are other improvements. As always, developers have been serious about balance and have continually made changes since the beta. It no longer feels like either side has a distinct advantage from the start (games were normally in favour of the Alien team), and that's key in keeping the strain that makes the game so great. The clumsy commander interface has been made significantly sleeker and more flexible. It's much more rookie-friendly, with rookies showing up on players' overviews (so experienced players know if somebody needs help), and there's a thorough tutorial video. Newbies can also explore maps off-server, so they don't forget that there are vents behind their backs in-game. In NS2, players get a moderate sum from resource extractors, but also get resources proportional to the size of the kills they get, as well as for helping to build things. This encourages good teamplay and performance, without penalizing players who perform well in a bad team.
Marines, historically the more difficult side to play in, have obtained a few new toys to even things out. These include roving miniguns that can be moved by the commander, adding new defensive options. It also includes mechs in later tiers that are the Marines' Onos equivalent and can really dish out the pain. Aliens, on the other hand, have had their movement improved substantially, making wall-walking and jumping easier than ever. Some of the abilities available to the Aliens, such as Blink for the Fade, are awesome - with proper execution, a Fade can terrorize Marines as they try to catch sight of the Fade using this ability - and the new visuals help.
Overall it feels like the developers of NS took what they had and turned it all up to eleven with better graphics, better gameplay, and more toys to play with. They haven't tried to change the original, successful concept. Given their history, it's certain that the developers will continue to release content and balance patches over time. Anyone who was a fan of the original game should try this, as should anyone who enjoys the Alien franchise or tactical shooters in general. For 30 bucks on Steam, it's actually hard to find reasons why players shouldn't get this game.
Now if you'll excuse me, I'm stalking a lone marine.