It doesn't include Games for instance, it will probably also lack Windows Live components, movie editing stuff etc.
I believe it's only about a 100MB difference. Things like included games are disabled by default. They're likely only decompressed and made functional when specified -- thus the small difference.
Features wise, Enterprise is identical to Ultimate. The only difference is that Enterprise is volume licensed and you can't find it on store shelves. Games are available on Enterprise, but they're disabled by default. Enterprise has everything Home Premium has, and more.
Using Ultimate is the right choice if you're not working for corporates Enterprise are using Volume licensing (Mostly for corporates) Ultimate using RTM/OEM licensing is for single use But RTM/OEM licensing are basically a joke....since we introduce the 'activators'