To me, the draw to Ghost Recon comes from its special flavor of tension, mixing good controls with the ability and need for strategy. Ghost Recon mixes your strength with your vulnerability. Your soldiers are not superhuman, not unkillable main characters, they just happen to be the people at your disposal.
The game itself provides a backstory on specialists, but for the most part you are filling in the blanks yourself. As you play, you grow more connected. And your people die. But not an overly dramatic death like, say any Call of Duty main character, but yet not something quick and painless either. Most of the time, you made the decision that got him/her killed, and you have to deal with it or retry for a better outcome. If you choose to push forward, you sometimes end up thinking what you could have or should have done.
Gameplay relies on team tactics as much as personal ability. When you have the advantage, it means something. You become the monster in the closet, what any bad guy fears the most. But when the tables are turned and you don't know where the enemy is, you feel very vulnerable, because after all you are just as easily killed as everyone else. It lends itself for cautious scouting and tense firefights. The planning and team commands have depth, and the actual controls are fluid and responsive. That is a big reason why the game still holds up today.
When I got my chance to play finally play paintball, I noticed that the the tension of moving through the underbrush, trying to get the drop on the other team was almost an exact mirror of how I felt in the game. Powerful and tactically flexible, but just a mistake away from being eliminated. That experience made me find new appreciation for the game altogether.
Few games have anything close to this feel, among them Splinter Cell:Chaos Theory and the X-Com series. But only Ghost Recon has ever balanced them so well.