Characters were always way too happy (excluding Episode 2 obviously) and for some reason they all feel like comic relief with their quirky oh-so-sweet personalities. Barney is a moron. I want to shoot him in the face every time he opens his big fat mouth.
Scenery was really boring. Nothing out of the ordinary, just warehouses, appartment buildings and a beach. Even the Citadel had some terrible art direction: big black square walls? Really, Valve? The driving sections suck since they're interrupted every ten seconds by puzzles or some arbitrary gunfight to go press on a button. Level progression couldn't be any more linear, to the point where finding that one pile of crates that lets you climb to the next part of the level is infuriating.
Story was awful. The cutscenes are completely irrelevant most of the time, to the point where some of them basically turn into "OH LOOK IT'S GORDON FREEMAN PLEASE SIGN MY GUN I LOVE YOU" and maybe some actual plot once in a while. Basically the entire story is just "Aliens invade. Go kill the bad guy at the top of the big bad tower while helping the resistance".
Guns were insanely dull. They feel so damn weak, inaccurate and cheap, with the exception of the gravity gun.
The game is only pretty in terms of texture quality and physics. Valve has been using the same explosion model for years for every single explosion in the game and it's always looked and sounded awful. The magnum has no use, the crowbar's hitting animation is just plain weird.
But the worse part is, there is no ambiance. No music. No atmosphere. You're just this guy, killing aliens, along with some other guys. There is never any sense of urgency and barely any emotion.