For example:
* Checkpoints respawn full of enemies and you never manage to fully exert control or supremacy on the world.
* In world map that you have to look at in the game space and time continues while you do so.
* Vehicle physics are punishing and you cannot press a button to "fix" them. If you crash that jeep, it's done.
* Weapons constantly jam and are generally crap to use.
* Thrown weapons have physics - ie: grenades roll down hills (often back towards you)
* Plotline and factions are hazy and undefined. You have a core mission (hunt down an arms trader) but everything around this feels directionless and punishing.
* The game does not do much or any "fudging" of the systems to increase player enjoyment. Often games will silently assist the player to make the game more exciting (your health is not linear and goes further as you approach death, enemy and your own aiming of weapons is improved or reduced to highten tension, combat arenas will be arranged to be more "fair to the player) and Far Cry 2 ignores this.
I would hesitate to call Far Cry 2 a "fun" game but it is certainly a good one, and has spawned a lot of good discussion, like this piece and pretty much the whole of the Idle Thumbs Podcast(now on extended Hiatus, sadly).
(Edits to fix formatting)