Instant on: Your client does not download the blockchain. It uses a network of specialized servers that index the blockchain.
Forgiving: Your wallet can be recovered from a secret seed.
Safe: Your seed and private keys are encrypted on your hard drive. They are never sent to the servers.
Low trust: Information received from the servers is verified using SPV. Servers are authenticated using SSL.
No downtimes: Your client is not tied to a particular server; it will switch instantly if your server is down.
Ubiquitous: You can use the same wallet on different computers, they will synchronize automatically.
Cold Storage: Sign transactions from a computer that is always offline. Broadcast them using a machine that does not have your keys.
Reachable: You can export your private keys into other Bitcoin clients.
Established: Electrum is open source and was first released in November 2011.