The environment is what got me hooked. I saw the cover to the Planes of Chaos boxset and it made my imagination go wild...it looked cool.
I love the idea of being able to travel to whatever environment I feel like adventuring in no matter how crazy it is. It's also easy for me to get players from one environment to another as quickly or slowly as I want.
I like how I can run
any type of creature I want to throw into the game. I even get to use creatures from any other campaign setting if I wanted to.
I like how I can also run material from other campaign settings if I want to. If I got an itching to run an adventure in the Underdark and encounter Jarlaxle from the FR setting, I can easily do it.
The weirdness of the planes is great. I like how a PC can change for better or worse just depending on his alignment or other factors. For some reason I find it fun making PCs adjust their adventuring gear to better suit them for their environment...or change their spell list to help them adjust
The official NPCs from the Planescape setting are very interesting....Uncaged: Faces of Sigil is a great sourcebook.
The factions help spice things up a lot. They help give more character to PCs and they are great tools for building plotlines.
When I imagine the PS setting in my head, the visuals are also another reason I love the setting. Thanks to the artwork in the books, its helped add a flavor to a world that is a lot different than what we normally see. If the right person made a movie based in the PS setting and used the same artistic style as the movie setting, it would be very cool to watch.
I really like the more grimey atmosphere.