Newbie here, quick question(s)

This is a hard question to answer. To TL;DR as much as possible, I'll list some major factors influencing the "viability" of a 'Mech.

Geometry: Scaling, surface area and volume of the 'Mech, frontal and side profiles affect the viability of a 'Mech greatly. Flat chested and humanoid 'Mechs tend to tank extremely well, while long-nosed 'Mechs tend to pop a side easily.
Hitboxes: Hitboxes affect how easy it is to roll damage in a 'Mech. While a 'Mech with bad hitboxes can still be viable and meta (the CTF wasn't the tankiest 'Mech but it was wildly popular for a while) having both firepower, mobility and tankiness is still extremely important.
Hardpoints: Hardpoints make or break a 'Mech. The quantity, location and the "density" of hardpoints affect a 'Mechs viability greatly. Generally you want to have a lot of very high mounted hardpoints. Energy and ballistic hardpoints are very desirable in the current balance state.
Mobility: Speed, torso twist range, torso twist speed and jump jets affect this. High tonnage but mobile 'Mechs are very desirable (TBR, BLR, BNC for example).
Quirks: Some 'Mechs receive better quirks than the others due to their weaknesses in other categories. While these quirks are meant to save them from irrelevance and offset their weaknesses, some quirked 'Mechs are extremely potent at skilled hands. These include TDR-5SS, WVR-6K, DRG-1N, BJ-1X and such.

So in order to be popular, a 'Mech needs to excel in a few of these categories. For example, the TDR-5SS is wildly quirked for Medium Pulse Lasers. With 7 energy hardpoints, it has enough hardpoints to boat these lasers(good number of E hardpoints, decent placement) mount a beefy STD engine and be fast (good mobility), it is extremely tanky (good geometry and hitboxes) and it has the quirks to have great firepower (the most important part).

/r/OutreachHPG Thread