The story is very immersive and connected to the gameplay. Instead of
detached cut-scenes, as is way too common in the genre, almost all the
scripted storylines are delivered by NPCs while the player is in full
control. (This even introduces a bit of interactivity to these "cut-scenes"
where, for example, an NPC might turn to look at the player while he is
delivering scripted lines.) While these scripted lines and acting are
delivered, the player is free to move and look around, so he can look at
the "cut-scene" from any viewpoint he wants. (Although one slightly
negative consequence of this is that the player may inadvertedly miss
something which he could have wanted to see.)
Tutorials are immersed into the gameplay and the actual settings of the
game instead of being completely unrelated events like in most other
games of the genre.
The sidekicks in episodes 1 and 2 work surprisingly well. They succeed in
not being annoying, not hindering the player, and in some cases even being
actually useful in fights. In this regard these games succeed quite
superbly in what John Romero attempted with his Daikatana but failed
miserably. (When I first played Episode 1 I thought "this is probably what
Romero had in mind when he wanted sidekicks in Daikatana".) Episodes 1
and 2 actually succeeded in making me fond of Alyx.
The graphics are superb and play at high framerates even in a computer
several years old. Valve has made especially good job at supporting HDRI
with shader model 2 graphics cards (which most other modern games don't
even bother to try).
The gravity gun.
The games are utterly linear. They are the definition of linear.
The only way they could get more linear is if the entire game was one
long straight corridor. One major problem with this linearity is that
it makes the games very short. While it took me something like 1 month
to reach the end of the main storyline of Elder Scrolls 4: Oblivion,
it took me 5 days to get to the end of Half-Life 2. Episodes 1 and 2 took
me 2 days each. While HL2 was a superb experience, its length was a
letdown. With episodes 1 and 2 I was already expecting short games,
but they were actually shorter than I expected.
One of the major thrills of Half-Life was its weaponry, especially the
exotic alien weapons. I was expecting more of the same with Half-Life 2,
but got disappointed.
The physics engine was not used enough for gameplay, it was little more than just eyecandy. There were very few physics-based puzzled, and even most of them could have been implemented without a physics engine. Likewise the gravity gun was heavily underused. While there were places which could only be passed with the aid of the gravity gun, they were still things which could have been implemented without a physics engine. In short, the physics engine was completely underused as a gameplay element (leaving it almost exclusively as a visual element).