Matewis: As in games where you play as a person stuck in an alien world, and in which exploration and discovery of the alien world plays a big role.
Currently I'm playing Unreal again, and I'm amazed at how much I still enjoy its narrative which lays sprinkled about the game world in random books, fallen terrans' logs and computer stations. The nali themselves are also a treat, some not knowing who you are and cowering in fear, while others decide to trust you by leading you to a hidden cache of supplies.
On the downside, it also reminds me how much of a letdown Unreal 2 was :P
You should definitely try The Solus Project. It fits the requirements and also it's one of the very best games in recent years. The alien planet is beautifully crafted and the events of the game are pretty intense at times.