As for the rather rudimentary environments, I think this is somewhat deliberate. The restrained aesthetic stands in stark contrast to how the game looks when the action kicks off, making the many particle effects Monolith crammed into the game stand out even more. Bullets tear absurdly large chunks out of concrete and plaster walls, filling the air with dust and debris. Sparks fly from struck light-fitting and electrical equipment. Hit the game's "SlowMo" key and the details are emphasised even further. Bullet trajectories are marked by watery trails in the air, and everything appears to become that little bit more solid, that little bit more "real".
It's also worth noting that while visually they're rather dull, F.E.A.R.'s environments are far more interesting in terms of actual level design. Monolith use these basic squares and rectangles to create dozens of interconnected combat arenas, each containing multiple pathways through them, avenues with which to flank and be flanked, and platforms from which to gain a vantage point. F.E.A.R is one of the last games from that old-school of singleplayer FPS design, where the emphasis was on designing levels around kinetic, entertaining shooting, rather than acting as pretty locale for cinematic set-pieces.
The level design plays a crucial role in what is F.E.A.R's most revered feature - it's AI. F.E.A.R famously uses an AI technique known as goal-oriented-action-planning. In basic terms, this means the AI has a range of available behaviours that it can choose from in order to achieve an ultimate goal. But this only works in a way that it noticeable to the player if the AI has clearly available options. Hence, each combat sequence takes place in a carefully constructed location that provides the AI with those options, through which their decision making is clearly communicated.
There's also some clever trickery in there (as is always the case with good game AI), such as the soldiers announcing behaviour changes when they fall back or move forward. They also appear to communicate with each other, checking in with their squadmates and becoming agitated if they are the last person standing. AI is always at its strongest when the rest of the game is built to facilitate it, and F.E.A.R is a masterclass in such design.
That's what I like most about F.E.A.R, I think. Everything is geared toward making the shooting as fun and interesting as possible. Much as I love the shotgun, most of the weapons in the game are excellently designed. Even the basic pistol is tremendously satisfying. The horror elements, while noticeably weaker than the action, are important in how they offer a change of pace just at the right moment, preventing the action from becoming stale.
At a time when games seem increasingly headed toward vast, uniform sandbox experiences that feel a need to contain a little bit of everything, it's enormously refreshing to return to a game that focuses on doing one thing as well as is possible. As a result of this, although many of the game's secondary aspects have dulled considerably with age, F.E.A.R's dazzling, unpredictable combat remains as fresh as it did ten years ago.