Right so whilst reading some MechWarrior: Online Forums, I see that there has been a fair amount of questions regarding the Medium Weight Mechs; "How is the Medium mech supposed to fair against other weight classes?"
Well I shall give a general answer here. I shall also discuss a theory as to what each weight class mech is generally designed for. Note: These are all in my opinion and all theory. Also all I've read in that forum topic is the OP haha
A Light Mech is basically designed for Scouting and Harassing. That's all they are really good at. They don't have the weight capacity to hold a large amount of weapons. Nor do they have the armour to withstand punishment. But they do have speed. Most Light Mechs move at 100kph or more. And it is because of this speed that makes them a fantastic Scout and Harasser.
A Scout is as it says. Scout out the enemy so your allies know who and what they're up against. A Harasser is, again exactly as the word describes. If a Light Mech isn't scouting, they're Harassing the Artillery/Support units. Support Mechs can't do any damage when they're panicking over a Mech that's attacking them.
Heavy Mechs (Lets skip Medium for now)
A Heavy Mech is basically a heavier version of the Medium Mech. Yes the obvious thing would be to pick a Heavy over a Medium. They are simply fantastic. They have enough weight to carry a huge amount of weapons and Heat Sinks to dissipate the heat generated. Yes they are generally slow. But who needs speed when you have a massive amount of armour eh? Besides, a Heavy Mech is generally your frontline assault Mech.
Medium Mechs (Now we're back to Medium)
So if Heavy Mechs are so fantastic why take a Medium? Light Mechs are your Scouts and Harassers. Why bother with a Medium? Well because a Medium can combine both the speed of a Light with the firepower/armour of a Heavy. Why not have good speeds to dodge the enemies attack, AND have decent armour to protect you if you do wind up getting shot. In addition, you have the firepower of a Heavy (or at least, slightly less). You can dodge and still dish out the damage.
Medium Mechs are a very powerful Mech. They are versatile. They can fill the role of the Light Mech if need be, or that of a Heavy. They are essentially a "Jack of all trades, master of none".
Now these are your tanks. No really. Ok all Mechs are essentially walking tanks. So think, if Medium Mechs are your infantry, these are your tanks. Assault Mechs have the largest weight capacity allowing them to have a huge amount of firepower and have the armour to take it too. They are hindered by speed however. But hey, They actually do have the armour to take it! And take it they shall. And whatever damage you deal to them, they can give it back... at double or triple what you could've ever done. But they are slow... So they aren't invincible.
But when you think about it, this is what the tabletop game does. It gives each weight class a role. They have their advantages and disadvantages. If MechWarrior: Online is going to be anything like the Tabletop game, then this is what each weight class will be used for. Otherwise, it may just be like MechWarrior 4, where Assault Mechs rule the field.