My very first National Park Service job was working in Yellowstone in the summer of 2007. It was an amazing experience and it being my first time living outside of Colorado, it really gave me the itch to live in and explore other national parks here in the West.
As far as photography in Yellowstone, that first summer I was just beginning to get a taste of it. I have been back several times since then and I really enjoy taking photos of all the thermal features Yellowstone has.
I also highly recommend visiting Yellowstone in the winter. Old Faithful has a completely different feel to it than the summer, and has tons of great and unique photo opportunities.