1. The server is not open source. I asked the developer about it, see here: https://bitcointalksearch.org/topic/m.571018
2. The server needs to know you. It needs to store an account with your addresses, because it keeps track of your transactions as it downloads the blockchain. In other words, its database works just like bitcoind, whereas the Electrum server behaves like a Blockexplorer database
These two facts create a situation where users are tied to a server that belongs to a single individual.
Even if Jan decided to open-source his code, the fact that users need to be known in advance from the server does not favor privacy or freedom of choice.
This is the reason why I decided to write Electrum (https://bitcointalksearch.org/topic/m.585227)
This is the reason why I am interested in using Electrum.