It's not the Mechs.
It's not the Tanks.
Or the VTOL's or the Aero's.
It's not the maps, nor the game itself.
It's the Team Work which makes this game the most enjoyable thing for me since breaking myself on tree's as a kid or conscript spamming d00d's on RA2 all those years ago.
I had two games last night which prove this is my most enjoyable thing about this game. Played the first and team work allowed us to overcome a technically superior force and hold down all of the TC points and fight the enemy on such a front that each and every Lance or Demo-spam they sent out was utterly annihilated. It went so damn well, infact, that I still had the Uller C I started with around forty minutes in, because I didn't NEED to change Mechs and I was too busy making Scat's and Whammies go Critical on their torsos.

Cue the second game. Teams are mixed up, new guys come in, some previous players leave and it begins on TSA Sandblasted. I'm well in the swing of my Team Work kicks now, only to find me and two other players out of our entire team are working in co-ordination. The rest just leap, frolicking, into the killing ground in the centre in Ravens and Owens and Parti's, Oh my! They died. A lot. I did too, in fact, because around thirty minutes in when our team finally decide to make a ragged, somewhat stable firing line to combat the mass of sniping mechs on the enemy team they fail to notice the two Warhammers approaching our base. From the right. Directly in line of the hanger exit. Which you can see. Like two big giants of PPC doom in your face.
So, me in my valiant little Puma proceed to harry the Whammies, letting everyone know we have not so friendlies right outside our spawn. Not a single response, nor does anyone deviate from facing the enemies half of the map and pot shooting at rocks. I think it took me a while but I managed to corral the Whammies away with repeated laser fire and just generally being a nippy little bastard.
By this point, the team has managed to get a few kills under its belt, but nothing compared to the utter raping we were taking. So, a few more, impressive components could be fielded. We had LongTom support. I got myself the absolutely beautiful LRMX20X4 'Vulture' and began pounding the enemy Madcat sniper like whoa. Racked me up around 160k in total just by LRM's to the face, helping soften up our opponents for the other guys to sweep in and take the kill.
Except no one really swept in for the kill because it was a team of utter Lone Wolfs.
Then we got an Atlas....who never moved from more than 300 metres from the base spawn and sniped from over the dunes.
Cue the invasion of two MadCats and two Whammies, straight into our faces from exactly the same place as the other two were before. I tried to keep moving and holding them down with my LRM's. Kept taking an utter pounding and running to base, repair, head out, LRM some more, get more C-bills, repair more Vulture before rinse and repeat.
The Atlas hid behind the defence lasers, the tanks all ran back into the hanger to hide and the light mechs decided the other side of the map was the place to be. This resulted in the Whammies sitting behind out base, out of sight from the defence systems and utterly pounding the shit out of anything that poked its head out. Now, I died. A lot, that game but it still didn't stop me buying a Scat, jetting out of the hanger, over the Whammies and start harrying them to get them away from our base, leaving perfect opportunity for our boys to smash them to shit from behind.
So it turned into a screaming, jump jetting me and two MC's and two WHMR's. It was not fun. Managed to hurt one Whammie and piss off a Madcat before I had to eject.
Cue my BA rampage. I did more total damage to the Madcat that didn't have to rtb for ammo and the two Whammies than the freakin' Atlas did. The only guy who was helping properly to clear our spawn of these invaders was the damn LongTom. So I thought 'frak it' hitched up a Puma and rode out to glory in the final minutes of the game. It was then, and only then, Team Work ensued and a sweeping advance in a line managed to push our point of control back up to the pyramids.
Then it became a sniping cluster-frak again and everyone pell mell'd until game end.
Team Work. It wins games. It doesn't matter if you can frag 15-0 on your own, congratulations you're a hero. You still loose if your team hides in the hanger and doesn't suck it up and go 'This ones for you, Jesus' and go out the main entrance all guns blazing. You work together, heavies supported by lights and sniping med's, you have Aerial fly bys and AA tanks and you win that game like it's made for you.
You don't, and well....the above happens and it makes the baby Jesus sad.
TL;DR?
Teamwork. It's the other half of the battle.