Speaking only for myself, it's not so much Planescape hate, it's more just being really tired being told that the new stuff sucks because they don't properly follow books that have been out of print for over a decade. Who cares?
On these boards? It looks like Shemmy cares. I think he might be the only one that I've really seen, or at least the most passionate one.

Judging from his posts, it just seems like he doesn't like 4e's decision to cut ties with the older edition material 'cuz the older edition material was good for him and didn't really need to be supplanted with something new.
I don't get the impression that it's a very...large number of people who do care. Most people here (and most people who like 4e, quite obviously) don't. I mean,
I really don't, and I'm a deeply avowed PS fan.
It's the whole "They destroyed Forgotten Realms" crap all over again.
It's basically the same argument, but I wouldn't necessarily call it crap. It's totally valid to bring that up as a reason not to like the new stuff.
It just kind of surprised me, I guess, that so many posters come out with "I LOVE THE FACT THAT THEY DON'T MENTION PLANESCAPE AT ALL!"
....but still, they diagram the Great Wheel, they mention the Lady of Pain and Sigil, and one of the PP's might very well be a faction-relevant one...so maybe they did?
Does...does that mean you don't like it now?
I get being fed up with the argument (it is kind of an old one by this point, though that doesn't mean it doesn't deserve to be discussed), but I'm not so sure I get reacting to that exhaustion with something so...hostile? I dunno, I could easily be seeing hate where there is none, though.
The whole kerfuffle about the City of Brass being the oldest city is a perfect microcosm of the whole thing. If you got into D&D in the past 10 years or less, then Planescape completely doesn't matter to you. Why should we continue to be beholden to material that's been dead and gone for years?
There are a few good reasons to keep in the old stuff. Listing them would probably derail the thread, though.

It's a choice, with consequences, benefits and rewards. Just like ditching the old stuff. There's hardly a clear-cut right and wrong in the case. It's more of a preference. And part of the reason for ENWorld is so we can all discuss our preferences.
Judge the product on its own merits. If it sucks because there isn't enough information, or poorly edited or the art is crap, then fair enough. Books suck. But, claiming it sucks because it ignores Supplement 8321xY page 213 subparagraph 2 is a big pet peeve for me.
So do all these old-school references in the product (Mercykillers, the Great Wheel, Sigil, the Lady of Pain, Bladelings, most of the monsters) turn you off of it?
I get it being a pet peeve (minis do it for me

), but did you ever think that maybe disregarding a beloved past is a pet peeve for Shemmy (or anyone else who is constantly harping about the changes that I've missed)?
It's totally okay to love the book for doing something new, and it seems like it *is* doing new things. But the PS hate seemed a bit mean-spirited to me, a bit "I love the fact that PS fans are crying right now, so I will buy the book for Schadenfreude!"
I could be totally barking up the wrong tree on it. It was just a passing observation. And I did want to point out that I am one big PS fan who is okay with the changes so far (even though I can cite you Supplement 8321xY page 213 subparagraph 2 if I wanted to).