I've decided to take my time writing about Neal Adams. There are so many fine pieces about him on the 'net that no one needs mine right this minute. I think I'll wait and see what, if anything, isn't said about him that I can supply. In the meantime, I recommend the thoughts of Paul Levitz, Mark Waid and Roy Thomas.
One of the many ways Neal changed comics was that he somewhat altered the fundamental relationship between "The Publisher" and "The Talent." Before Neal came along, if you wanted to work in the field, you pretty much had to become an indentured servant to one company or another, allowing them to place you wherever they wanted you. Even Jack Kirby, who at the time should have had the clout to not do that, had to work on a lot of comics he didn't particularly want to do and conform to a lot of editorial dictates he resented.
But Neal came along, first of all, at a time when New Artists were a rare thing at DC or Marvel. They were quite satisfied with their talent pools…which is why it took some time and persistence for Neal to even get serious consideration at DC. And when he did begin working for them, he managed to make them understand that they needed him more than he needed them. He'd had a syndicated newspaper strip (Ben Casey) at age 21 and he had loads of advertising work, most of which probably paid way better than drawing for DC or Marvel.
I don't think editors then knew quite how to deal with this guy who could and would say no to work. But they recognized that he made a difference, especially in the area of covers. He was bringing something new to comics and most of them wanted a piece of that "new." It was especially impressive when Neal, drawing some of the Batman comic books, raised the bar and impacted all who were working on the character. And his influence spread to a lot of comics he never worked on at all.
Facebook is awash with tributes from artists whose careers began or were bettered by Neal — some by direct contact; others by just seeing what he was doing and being inspired by it. Check out some of those essays as well. It's going to take time to fully appreciate the influence of this man.