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Glenn Reid left Adobe to join NeXT, left NeXT to make NeXT software, and left that to join Apple just after Steve Jobs returned to the company, and brought with him what would become OS X. Reid was eventually tasked to build iMovie and iPhoto, and worked a lot with Steve Jobs on both projects. He's written up some of what that was like on his Inventor Lab Blog:
Not only did [Steve Jobs] know and love product engineering, it's all he really wanted to do. He told me once that part of the reason he wanted to be CEO was so that nobody could tell him that he wasn't allowed to participate in the nitty-gritty of product design. He was right there in the middle of it. All of it. As a team member, not as CEO. He quietly left his CEO hat by the door, and collaborated with us. He was basically the Product Manager for all of the products I worked on, even though there eventually were other people with that title, who usually weren't allowed in the room :)
Or as John Gruber put it on Daring Fireball, a designer with Carte Blanche. Contrast this with Microsoft, run by a sales and marketing guy, and Google, once run by a software politician and now a founding engineer, and other companies where design is given a backseat to all of those things, thanks to all of those people, and you get a sense of the difference Jobs made to Apple.
Tim Cook isn't a product guy. He's an operations guy. Time will tell what, if any difference that makes. He's got Jony Ive on product still, though, and if you've ever heard Cook talk about the staircases in Apple Stores or the finishes on MacBooks, you can tell he cares very deeply about the product designs he turns into product realities.
There's a lot more great stuff in Reid's piece. Go read it.