I honestly don't hate most of it. I use to prefer it, back when DVDs had me convinced that extended editions were better by default. But, I think most of it simply doesn't add anything of value. I mean, millions of people fell in love with the original cut, so that's kind of an obvious statement.
Things like the learning switch just seem pointless to me. Not bad, but not needed. Same with the T1000 scanning John's room (is this the one someone informed me isn't actually part of the extended cut a while back?). The dream sequence gets a little too premonition-y for me. Feels like less of a dream and more like Ghost Kyle actually visiting Sarah. Seeing Old (young) Sarah is done better later when we see Current Sarah looking through the fence at her old self and young John. Still kind of cool to see Kyle, I guess.
The only parts I actively hate are smiling Arnold, obviously, and glitching T1000. I think that straight up makes the movie worse. I like the T1000 being absolutely unstoppable up until the final moment. Fuck him glitching out throughout the entire final stretch. I don't like it. Gives a glimmer of hope that takes some of the tension away.
But, yeah overall I am mostly fine with it. I more just like to point out that it is not the directors cut; the theatrical version is. It's a pet peeve of mine, because I am crazy, and it's my favorite movie ever.
EDIT: there is more Miles in the extended cut, right? I might actually like that. Maybe I need to rewatch it.
The part with the T-1000 scanning his room was one of the still-excised scenes that were added in the Ultimate Cut version, along with the alternate happy-sappy ending where JD doesn't even happen. I'm not a fan of either thing because why?
A lot of the smaller pieces added for the DC/EC version (Cameron actually approved both of this and the TC, just prefers the TC) I rather like just because a couple of them tighten small continuity up a tiny bit. Super small things like the T-1000 stealing the cop car, him killing the dog and checking his collar, etc, just add tiny bits.
The scene with Dougie and the other nurse shocking Sarah to force her to take her pills just adds more "these guys are assholes" to it, so it's a take it or leave it. You're supposed to hate them anyway.
I actually really like showing them reprogramming the T-800 a bit, mainly because Sarah trying to destroy the chip is totally in character and one of the moments where her (well-founded) paranoia could have cost them the ability to defeat the T-1000. It was silly to have to unlock its ability to learn, which was the leading-in point of the scene, so... this one is tough to defend when the lead-in is dumb but the main reason for the scene in general is great.
Smiling Terminator is really weird, yes. The talk he and John have while grabbing ammo, OTOH, is not.
I'm on the fence about the glitching, though. I'm with you that it's good to think of him a some otherwise unstoppable machine, but showing that he does, indeed, have weakness is also good.I think one of or two of those small things being gone is fine, as the one with his rippling was left in anyway, but the one brief shot where John looks at the fake's feet and is more easily convinced of which one is real is good because it's better than taking a super shitty gamble of which Sarah is real or not.