This thread is expanding at a ridiculous rate. It will soon encompass the known Internetz.

Half-Life was the first game to really do a good job of telling its story through events in the gameplay (using heavy scripting) rather than delivering expositional lumps like Doom level screens or Marathon's (excellent) terminals.
Which was part of what I consider to be the ('seemingly' seamless) level-handling. That being said, there was no real story. There were many insignificant events which had no impact on anything outside of being the red key-card.
Honestly, the only real parts of the 'story' (from what I remember) were: Explosion - > Generic Scientist No 4. tells you to get out - > Ant-Lion-Manticore-Thing - > Spec Ops Soldiers come to kill you - > Save some scientists - > Go to the fifth-dimension-thing and kill the boss - > Be the train. You were completely railroaded on a quest to reach the finish line, where nothing you did felt like it actually mattered.
And there was no character development. Hell, there were no main characters
in the entire game. G-Man came the closest, but still failed.
The fact that you learn more about what happened in HL1 from HL2 should be telling enough.