I'll try to keep this short, I love this game because its gameplay is very deep but rewarding. There's always mech to master and a build to craft for a very specific purpose. Some people cling to the same mechs and builds and get their kick out of winning consistently, but that's usually when I get bored.
Also a big factor in why I love the combat so much is the generally high TTK. Unlike other twitchy FPS' where only split second reflexes decide who wins or loses, MWO allows you to make mistakes and recover from them. The trick is to make sure those mistakes do not add up during the combat. Whoever consistently makes mistakes gets eliminated pretty fast during the matches, while those who have learned to avoid them live long and pull kills and damages way above their weight. High TTK also brings many many little decision making factors in; like positioning, absorbing HUD information efficiently, analyzing builds on the fly, spreading damage, keeping track of enemy positions, knowing when to commit and disengage and so on. All of these little factors take a long time to master but ultimately they're very rewarding. I've been playing for over 1.5 years, but I'm yet to master everything.
This is also why I frown on people using builds which allow very low TTK with good aim, which suddenly makes these little factors mostly irrelevant. Though, Paul has recently taken promising measures towards addressing this issue.
I wanted it to keep it short, but there you have it.
TL;DR: I love it because it's easy to get in but hard to master. High TTK allows the one who mastered the most areas of the game to come out on top regardless of the situation, and trying to achieve that is fun.