I have two pairs of basically the same headphone except one has pleather pads and the other has velour pads.
I know people have said that the velour pads give you a little less bass, and make the headphones sound a little more like "open" headphones, and reduce noise isolation.
But the degree to which noise isolation is reduced is well beyond what I would have guessed from the descriptions and reviews I've read - the extent to which I can hear people and sounds around me, while wearing the phones with the velour pads is more like to NOT WEARING headphones than like wearing the pleather pad headphones.
And, of course, as many people have mentioned, the headphones with velour pads "leak" sound a lot more. My kids said "I can hear what your listening to from here! (across the room) - they thought it was remarkable how loud the music sounded compared to when I was listening to the other headphones.