Gravity gun and use of physics that never feels like a gimmick!!
Variety of gameplay.
It did annoy me how the console versions of the Orange Box got an updated engine for ALL the games but the PC version did not! But then the PC version has plenty good quality mods like using the Portal gun etc!!
http://www.hl2mods.co.uk/find.php?genreString=&modeString=Single+player&downloadString=&
Cinematic MOD looks real nice.
I'm gonna have to try that out. 9.1GB for the full pack!!
http://www.fileplanet.com/196013/190000/fileinfo/Half-Life-2---FakeFactory-Cinematic-Mod-V9.5-HD-Mod-Complete