They were incredibly innovative games and had unmatched storytelling and gameplay for their time. They were highly praised by both critics and gamers alike and pushed the genre forwards. The same just isn't true of games like Call Of Duty, RAGE or Crysis, which have their merits but failed to go the extra mile needed to become classics. Far Cry did a great job technologically but had a pretty poor narrative. Battlefield 3 had exceptional graphics but the singleplayer game was on-rails and had a clichéd narrative. The only games that have truly competed with the Half-Life franchise over the years - narratively, technologically and gameplay wise - are Deus Ex: Human Revolution and Bioshock. That's just in terms of singleplayer games though. Multiplayer is an entirely different affair, with classics like Unreal Tournament, Quake 3, Borderlands, etc.
Let's not go down this tired excuse of defending games. Most people are going to play on normal difficultly - hence why it's called "normal". More importantly, people should play a difficultly that reflects their gaming ability - if you think a novice gamer is going to enjoy Delta difficulty in Crysis then you're delusional.
Gameplay is only one aspect of a singleplayer game. It has to be combined with enjoyability, storytelling, pacing, interactivity, etc. Some parts were well done but it certainly wasn't even close to the quality of Half-Life / Half-Life 2.