So after three pages of replies and opinions, there are a few things we can probably all agree on.
1. You can't see and do it all in one visit. Brain overload. But you'll be back.
2. One lap of the Grand Loop is about 140 miles. You don't want to do it in one day. With buffalo jams and traffic, you can't do it in one day, so relax.
3. Spring is different than summer which is different than fall. And you can't do winter without a snow machine and permission.
4. In any season, you can go from cold to chilly to hot and back in the same day, so dress in layers and comfortable shoes.
5. Camping options inside and outside the park range from no amenities to FHU with cable and WIFI.
6. The animals are wild. Yellowstone isn't a petting zoo, so don't try. Other than a field mouse, every animal can hurt or kill you if provoked.
7. If you're 62 or older, you can buy a Senior National Parks Pass for $10. The best bargain you'll ever find. Its good for life, and you can buy it at any National Park. Good for the car load or up to four people at a walk in.
Go to "www.nps.gov/findapark/index.htm" to find a place close to home to buy one.