As you progress through the game you can train and promote your soldiers to make them more effective in combat, unlock new abilities and upgrade their weapons armor. The interesting part of it is if a member gets injured (which is really easy to happen on the harder difficulties) they are not available for the following missions until they heal. So if you only upgrade one solider and he/she gets injured, you're in a really tough spot.

The weapons, abilities, items and armor you can unlock are varied and interesting enough to make it really fun to progress through the game and build a custom squad based around how you like to go about your missions.

You're always against the clock in one way or another. Globally your enemies are moving forward with their plans and unless you actively work towards countering them, they will win. You'll be short on cash, manpower, and will have to work with inferior tech. On the field most missions have turn limits forcing you to act aggressively. This scrambling for resources, making with what you can, racing against the clock creates great suspense that makes it all the more satisfying to win an important mission.