In addition to those already mentioned:
500 mile minimum earning per segment
Better saver award availability
reduced close-in/change/redeposit fee on award tickets (United Club card eliminates close-in booking fee)
Instant Y/B upgrades

What you get from the Explorer card that you don't as Premier is two club passes. Hard to find anything else. The bonus miles (2/$1 on United) can be had from several cards offering 2pts/$1 on travel (Chase Sapphire Preferred, for example). Of course the bonus miles for signup for the Explorer might make it worth getting anyway.