Oh whisper my name and I'll come calling...
It intimidates some people. Either from the 1st edition 'the planes are where gods are and any attempt to detail them or go there ruins a mystery that is so mysterious that we can't talk about it'. Obviously they've never read the Planescape material in depth.
And then there's the mindset of 'it's only for high level people. We'll get killed! Waaah! Let's go play something safer like Tomb of Horrors!'
I think both of them are misguided personally, but for the 1st one it's more a differing philosophy in how you treat the planes. Obviously I'm a firm adherent of all things Planescapish and I feel that planar campaigns don't water down the mystique of the planes, but if done properly they increase that mystique and give you a firm appreciation for it.
Millions of Tanar'ri and Baatezu swarming like red and black ants across the ashen, bleak landscape of Oinos, living and dying under the shadow of the Wasting Tower of Khin-Oin, a 22 mile high citadel carved from the spine of a god. Shapeless, formless entities locked within the ice of Cania or swimming unseen beneath the bottomless mire of Minauros. A fiend redeemed through a single spark and imperfection of good, finally after millennia escaping imprisonment by his fellows and making his way to Elysium where when he steps through the gate and the sunlight touches him, his former form melts away in the sunlight to leave him in the form a lesser guardinal; redeemed in every way possible, and he weeps in happiness.
Does that lessen mystery or does that leave you wanting more? A properly run planar game should evoke grandeur, mystery and the manifest extremes of alignments and philosophies. The planes aren't just big extraplanar dungeons to go kill higher level monsters for more kewl loot and gain mad XP. If they are, please get off my internet, now.
The planes are the planes, and I don't treat a planar game as hopping between prime worlds that many folks seem to dislike. The planes are the planes, and you've got more than enough going on there to treat them in a campaign unto themselves.
Check out my storyhour(s) if you want an example of my approach to this all.