Earth Siege 2 was a good game, but the reasons MW2 seemed vastly superior (for me at least):
1. Smooth animation. ES2 just felt really clumsy. MW2 was very smooth, and responding well to inputs.
2. Attention to detail. Let's face it, not even games today show the same effort to pay attention to detail as MW2 had. When you destroyed enemy mechs in a vacuum, there was no fire, they just broke apart. Eject and you died. You could identify, by name, nearly every enemy mech pilot, or in Clan Wolf's 3rd mission, Blade Splint, identifying many of the buildings revealed a lot of easter eggs.
3. Immersion. I don't know the backstory of ES2, and they don't cover it very well throughout the game. MW2 on the other hand, fully immersed you in the universe. New players, like me, were incredibly confused about the weird things the trainer was saying, and I looked into the clan hall projector for information. Holy crap! I read EVERYTHING. History, layouts, culture, language, etc. The situation briefs and debriefs also helped immerse you in the universe, by letting you know the status of the war.