UFO Enemy Unknown:
Defend the world
Build any base in any location
Three-button aircraft combat
Strategic balancing of pure research, quickly adapting alien weaponry, constructing good gear but not having enough, manufacturing just to make money, and actually having the immediate resources to go out there and keep the world together.
Agents are human who become better with experience.
Tactical turn-based combat
Pretty much anything can be destroyed if you brought enough firepower.
Tanks and landed aircraft on both sides.
X-Com Apocalypse:
Defend a city
Build in one of a limited number of specific buildings with pre-designated entry points
Useless suicide-can ground vehicles, aircraft that generally just hover and trade blows, but agile craft can work for the first week or so...
Strategic system slightly simplified, due to overpowered machine pistols, ultralight armour, and lack of alien rankings.
Agents are human, robotic, or partially alien, who become better with experience, but there is a limited quantity.
Tactical turn-based or simultaneous with pause button.
Quite possibly the pinnacle of destructible environments in computer games.
No tanks, landed enemy aircraft only
Predictions for the future:
Defend one guy, maybe a squad.
Be given a default base.
full-blown car-driving simulation included, but get out of the car before combat starts.
Choose which gun you want next.
One agent, but you can mutate them and install robotic parts.
Twitch-based continuous combat.
The whole world consists of amorphous blobs.
Fully functional alien aircraft, which just disappear when you manage to kill them with a handgun.