I am kindda new to this community. And I am one of those who came with Steam (but I tried MWO before, not much, but I did).
Currently I have 300+ hours of gameplay and about 900 QP matches played as statistic says (and some CWs - my guess is about 50-60).
So I want to lay down some thoughts on tactics in MWO. It is not a dogma, just my thoughts. If I will not help you it will at least help me to structurize some things.
Let's start with tactical roles. As I can think of tactical role in general is how pilot pilots his mech on battlefield. There are several tactical roles I can think of (one can mix them of course):
1. Tank
2. Biter
3. Defilader
4. Backstabber
5. Scavenger
6. Sniper
7. LRM support
8. LRM striker
Tank
I guess everything is clear here. You move strait on the enemy and shoot all you have at him. It is more essential not to have tons of armor but to have weapons that you can shoot non-stop. ACs combined with some low heat lasers or SRMs are the best for tanking.
Assault mechs are more suitable for tanking because of armor and limited agility but one (if clever) can tank using even medium mech.
Of course when you develop some tanking loadout you take medium-short range weapons, balistics mostly. The thing you don't need here is high heat energy load that makes you overheat just after few alphas.
One can use chainfire lasers to tank but it is IMO much less efficient then ballistics load on that shore.
Moreover as a tactics tanking is good when your enemy is cored and you can finish him with some balistics or SRMs. That is why I like 2 or 4 machine guns equipped on my high energy loads like HBR-Prime (2 LPL + 3MPL + 2MG) or Nova-S (6MPL + 4 MG). They heat as hell so tanking is no good for them in general, but when the enemy is cored and close - I usually tank-finish him with MGs - which gives me time to chill too (Valid if I have no risk of getting shot by his teammates around).
Tanking is good because it is rather simple to aim when you move strait to the enemy but it requires some skill just to know when and where it is OK to tank. Especially if you are driving some heavy and slow assault mech.
Biter
ACH pack all with 6ERSL on a battlefield is great example of biting tactics. You run around, always on the move, always shooting someone.
Not only lights utilize that tactics. I saw a lot of HBKs or WVRs doing so rather good. Most CDAs and IFRs use that tactics too.
The important thing here is to have some speed and agility to dodge enemy fire. And another thing - the smaller mechs are obviously having some advantage.
General loadout rules here are close to tanking, but you may have some low-mid heat energy load or use chainfire. Chainfire may work well if you shoot-non-stop and keep running around and avoiding massive hits.
Biters should fear LRMs and Streak SRMs - so it is better to have some cover nearby.
Usually biter loadout do not do massive alpha damage (or do, but to bite you need to use chainfire).
Be advised, biters are usually good if there is only one enemy and one or more biters. One biter alone meeting several enemies is dead biter.
Biters are good in a massive brawl between heavies - when everybody is busy hitting hard 1 or 2 biters can be of help and change the course of a big brawl.
Defilader
It is a basic hit-and-hide tactics but most of the time noone really thinks how one should use it to his advantage.
First of all defiladers should have strong alpha strike in midrange. As for me 2 best loads for defilader are: MAD-BH with 7ML + AC10 and TBR-A with 4MPL + 2LPL. Both do massive alpha strike damage in midrange but overheat in 2 shots.
No real point in being defilader if you have several AC2/5 or other weapons that do not do 1-hit-big-damage.
The other important thing for defilader is how you pick a position. First of all, do not move out of cover to make a shot if there are 2 or more enemies matching your weight waiting for you to come out. You will be overpowered and dead before you even kill 1 enemy .
If you see that 2 or more enemies are waiting for you to come out - try to find another position or corner to lean out.
Best defilader will take the position on flank of brawling teammates and shoot enemies that are busy attacking your tanking teammates.
It is best to change the position from time to time, especially if enemy already knows you are there and ready to strike back.
Week point of defilader builds are that they can't tank back properly if one heavy enemy tanks them (no good rate of fire due to high heat and/or weapon specs). So if you see that some undamaged heavy tanker moves to your position - it is better to move closer to your teammates.
Backstabber
It is quite obvious that backstabber is some fast flanking mech, shooting enemies backs. One need to have some skill and some luck to flank enemy or a group. ECM is a big advantage here.
Backstabbers build should have good alpha strike close range weapons. Best examples here - 6ERSL of ACHs, or 4SL + 2MG of LCTs (Pirate's Bane). SRMs do good here too - like 2 SRM6 + 2MPL on MLX-Prime. MGs fit too in combination with some armor-piercing lasers or SRMs.
The most important thing is that you need to do good damage to the back of an enemy fast and then run (in case other enemies saw you backstabbing).
Needless to say that backstabber is moving unseen before he kills his victim.
To be continued...