HL2 introduced large scripted events that happened without taking control away from the player - it upped the ante compared to HL1 and basically set the standard, everybody was flipping their lids about it at the time. I can see where people are bored of that now, the COD games are almost completely scripted events.
i dunno. being a pretty avid HL player including some of the more script-heavy custom levels, i guess hl2 just had almost no impact on me in that area. The only thing that hit me as unique was the grav gun tbh.