Here's the deal: Without even going into the Jedi arguments. They basically took a game that a lot of people liked and changed the game GUI and the way commands were issued. I'm not good at ELI5, but I'll do my best.

Pre-CU or Pre Combat upgrade: You could issue button commands in macro's including fight commands and have a whole fight choreographed before you got into battle.

There were 3 bars for the normal player like hp bars, one was a health bar, one was an action bar and one was a mind bar. Certain weapons and skills used the action and mind bars. Where as health that was based on your hit points etc.
However what made the pre-cu nice/pissed people off was the small fact there where attacks/skills that you could use to attack the health and mind bars. There were two special attacks that could make your character useless until you went and saw a certain profession to have them fix even after experiencing character death. Random drops also made some weapons too powerful like mind fire with insane damage that would do exactly that. That was the small piddly stuff. The big one was Jedi.

CU was just lame and wasn't around long enough for me to actually hate it, but it really did look like they were trying to make the GUI/Gameplay like WOW, which I wasn't really fond of.

NGE or New Game Enhancement was decent with the fighting and it was somewhat balanced (we could nitpick that all day) I really could go on all night about the PVP and the PVE, but it just wasn't up to snuff anymore. Loot became WOW grinds for piece sets and for PVP most people had to play at certain areas in order to get certain points and items. I guess when it comes down to it, by the time they threw the NGE at us, we were already frustrated with the CU and Pre-Cu.

I know I didn't answer all your questions, but suffice it to say it was like playing 3 different games when all I wanted to do was play the one I bought.