In which ways is Earthcoin different from Bitcoin? Also, the name Earthcoin suggests to me that it may be somehow more sustainable with regards to the computing resources required to support it; is there any basis in that? 

The official site appears to be http://getearthcoin.com/ which has some pretty logos but is woefully lacking in technical detail. As it probably always the case with an altcoin, it also has a thread on bitcointalk.org which seems to describe a very odd block reward system with some days paying 2 or 5 times the normal block reward.
As mentioned in the BitcoinTalk thread.


  Our goal isn't to compete, as much as compliment, create balance, and 
  not divert into launching 9 more coins because we can.


It doesn't look there is much difference. It may have a different block time, reward, and cap, but that is standard stuff to change for an altcoin. There appears to be absolutely no technological advance. It donates a percentage of coins to charity but that means nothing if it has no value.