OP made an amusing post.
Skill, depends on the individual. Mentioning this because of what other people were saying.
Off the top of my head and in my opinion, it's a combination of the following. I'm sure I've missed some things though:
#1 Aiming
#2 Predicting movement
#3 Knowing where to position yourself
#4 Knowing how to avoid damage and how to spread the damage across your mech
#5 Patience, and knowing when to be passive or aggressive
#6 Prioritizing
#7 Teamwork
#8 Being aware of your surroundings
Personally I find aiming in Mechwarrior Online to be more difficult than some of the more fast FPS games like Tribes Ascend. Then again, Tribes runs smoother on my computer than MWO, at least when I was playing it (having 3-5 people shooting at you at the same and being able to kill them makes you feel like such a badass in that game, yay for good aim). You can't instantly lock onto people with your cursor in MWO, it takes a while for your mech to catch up with them, and a bunch of things can happen in a few seconds.
Predicting movement is both important for positioning and the actual combat. You don't want to get caught out of position and get focused, and you don't want to miss your shots when shooting people.
I'm pretty sure I don't have to say anything about the importance of taking cover, and knowing when to bumrush a single enemy that has no support from his team.
Any sort of dodging is always good, though you're probably not going to dodge in an Atlas unless you're dodging someone with terrible aim/bad computer, so this applies to the faster mechs. However, knowing when to Torso twist is much more important to the larger mechs (Light mech arms in general are too small to shield themselves from anything).
Patience is something that is incredibly difficult for most people in many games. Of course, most games have a respawn mechanic, so it's pretty easy to want to just kill the enemy player even if it means you die. Being too passive will put too much pressure on the rest of your team, and will end up with you being outnumbered later. Being too aggressive leads you to dieing before you can effectively contribute to the team. Striking the right balance is something that's really rare.
Prioritization, should you be shooting that guy with little to no weapons over there, or should you shoot that guy to the side of you with a cherry torso?... Should you shoot the arm with weapons in it when it has no armor, or should you go straight for the almost untouched CT? Should you try to save the 1 or 2 allies that strayed from the group so they can help you later in the main fight and to remove the enemies, or should you stay with the main group to increase your chances of survival so you can help more in the main fight?
All sorts of things like this, but there isn't always a singular rule for prioritization, sometimes it really depends on the situation you are in, and you need to think quickly to properly act to the situation.
Teamwork is OP. However, actual teamwork is rediculously rare in video games, even if it's a game focused on teamwork. Hence why some games have built-in game mechanics specificly to make teamwork easier. If you've ever been focus fired, you should know how deadly teamwork can be. However, having someone scout the enemy out is nice as well.... Information is ammunition. You're not just helping yourself, you're helping everyone on your team.
Yes, memorizing the map layout is important in any game, but that's not the only thing I'm talking about. When people start engaging in combat in any game, chances are they will get tunnel visioned because it can be hard to multitask. So they may not notice that more enemys are going towards their position on the minimap. Or maybe that there is an enemy 1 hit away from death that is wrecking an allied LRM boat, but the player is too tunnel visioned . Remember the minimap! People exploit others being tunnel visioned all the time. You maybe the best 1v1 duelist ever, but if you don't retreat when enemies are inbound on your position, you will die.
In general, I feel all mechs take around the same amount of difficulty, but use different skillsets depending on how heavy/slow they are and what weapons they choose. With that said, the only weapon I can't really defend is Streak SRMs.
Personally piloting a heavy mech with LRMs (with lasers/ppc) is the most fun for me.
I tend to stay rather close to the front lines. I like to be 300-450m away from the closest enemy. It makes the enemy very tempted to try charging forward to kill me, without letting get too many glimpses of me (excluding when I have jumpjets). Also people can't avoid LRMs when you're that close. I also get to TAG enemies for anyone else using LRMs on my team as well (and ECM is a pain in the arse so I gotta be used to being near the front for those matches). Lately I've been getting several people have been focused me down so hard in almost every match, they'll just ignore teammates and go straight for me like they were going for revenge or something, which ends up having them dieing fast while I retreat with not too much damage because I know how to torso twist and use cover.
Looking at Sarna, Clan LRMs are more focused towards direct fire. I look forward to seeing PGI's interpretation of the Clan LRMs. If they are direct fire, I will use them even if they aren't effective (as if being effective or not stopped me before). I just need a Clan mech that makes me go "MUST HAVE!" because of it's design and has Jumpjets + Missles (unless it's the Mad Dog, I can overlook the lack of jumpjets for the Mad Dog).
Haven't tried doing an LRM boat Assault yet, I don't really trust my teammates that well (Practicly PUG only here), and I hate being slow.