Because everyone's all up in arms about things, here's what's to come in simple terms.
1.) Engine desync
Engine X no longer provides turn rate Y, accel/decel rate Z. Instead, turn rates are tied to the chassis. Accel/Decel are functions tied to the chassis, but always depending on Delta-V of current speed and top speed. A high top speed will result in a better accel/decel rate implicitly. And some mechs, such as the Phoenix Hawk, get their base mobility vastly pushed beyond their contemporaries in the base line stats. Which is amplified further by the skill tree, as that's a modifier to the base line.
2.) Skilltree Meganerf
We're all getting nerfed in firepower, and buffed in suvivabilty. The obvious effect is a deceleration of gameplay, and an increase in TTK (Read: killing takes longer, matches go longer). That's the basis for every mech. Don't get hung up on your specific favourite mech. You aren't losing X, everything is changing in regards to X.
3.) Skilltree IS blanket nerf
Again, everyone's getting nerfed, only IS mechs get hit
4.) The sky is falling.
It's not, the game is transitioning to a less restrictive (rule of three) system and on the way, some old things are being reworked. To reduce TTK, firepower needs to go down (happening), durabiltiy needs to go up (happening). Which, in turn, bridges the gap between IS and Clan.
Stop losing your marbles about all of this. Those "I WILL LEAVE" posts are pathetic. Stop clinging to something just because you are used to it, and adapt to the new system. If you are comparing it to the old, you will never find the clear sight to embrace the new. It's the same in every game that goes through a major overhaul. I can guarantee you, the top dogs are already equipped with a plan on how to approach the new system, and while you complain and moan, you'll be left behind and get stomped.
Read, understand, have a plan.