1. Legging I'll leg people when outmatched or they're using a cheese variant, mostly missile boats, maybe Novacats.
2. CAMPING Never valid. Very few situations in the game demand that you sit on your ass. Missile weapons are ultimate cheese and my god Arrow IV needs a huge nerf.
Defending from overwhelming attack force.
The best defense is a good offense. Thinking that you're being overwhelmed so you should hide in your base is what causes massive stalemates.
3. TEAMPLAY/TEAMSTACKING. Clan-stacking aside (go scrim in a private server), in any other game if one team is working together and the other isn't, the onus is on the other team to learn to play or deservedly lose. I don't understand constantly swapping teams to 'even the scores'. They don't always have to be matches won within 100 points of each other.
Here's what I often see: I'm on a team with a few other decent players. We're communicating status and targets, forming up into groups, and most importantly, pushing the frontline of the battle with appropriate mechs. Now, the other team is a bunch of decent players but they're not communicating and getting totally pushed back. Their first instinct is to turtle up - it's something they can do that doesn't really require working with others. So you end up with lots of missile and Arrow IV spam. So this goes on for a bit, and because the other team is a pack of retards, we're now 1000 points up on them or something ridiculous. So one of our decent players changes team.
Now all we see is a bunch of missile boats, and our ex-teammate being the only one on the frontline getting roflstomped by us. This happens to me when I (used to) change teams to even it up - I don't anymore because leaving a good team to join a shitty one just promises to ruin the next forty minutes of my life. Trying to corral a pack of obstinate idiots into getting into Warhammers and the like, stack up in groups, and call targets is a lesson in futility. Last night I was on a team and *every time I looked at the map* there was a stack of three or four mechs inside, or just outside the hangar. Our entire offensive consisted of one medium and two light mechs. Nothing I said would get them to leave the hangar so I eventually just ended up belittling them until the round ended, and made sure to join the team with the fewest of my ex-teammates on it as possible. A team who absolutely sucks but works together is way more fun than a team of good players who ignore everyone but their targets. Don't confuse your team's point lead and constant domination as a perceived skill stack, it could just as easily be the other team not working together.
That so many people aren't willing to work together is this game's greatest failing. If only in-game voice chat worked, that would do wonders.
4. PLAYER FAIRNESS My favourite is when a couple of enemy mechs try to dogpile you and when you fight back they run away. Yes, I'm talking about you Bushwacker pilot from Frostbite who was more than happy to RAC me while I fought an Owens, and as soon as I turned my attention to you, you turn around and fled back to your turrets.
5. SHOOTING ARMS I will happily hack the weapons off of anything with a Gauss rifle. Shadowcat C, Gaussdogs, Kit Fox As, and Hollanders. Played a game the other day where I think I blew off like 9 or 10 gauss rifles by the end of the match.
6. EXPLOITS What the hell is ballooning? Flying Aero when you know you're experiencing packetloss or have a ping over 100 is bullshit too.