This is a question a friend just asked me and I thought I would share my response.
Please feel free to correct me.
What makes the Steem blockchain so powerful?
Its a proof of stake as opposed to proof of work so no hardware mining and wasted electricity
The stake holders are voted for the community and there is mechanism to keep everyone honest
There are no transaction fees, instead transactions are rate limited depending on your balance
It has a block every 3 seconds and is theoretically able to handle 100000's transactions a block this allows for every post, vote and comment to be a transaction
To create an account it has to be funded to prevent account spamming. so someone has to put up the money to make a new account.
Steemit.com will create a free account for you but they are spending their Steem to do that, so they are pretty tight with verify accounts. You can create with the CLI if you can convince someone to fund it
It has all the primitives you would expect of a social network like posts, comments, votes so all the posts and comment contents are stored on the Blockchain, so is every vote.
Private keys take the form of a 52 character password from which you derive a set of other keys that have certain permissions
Your wallet address is the user name you choose. Transfers from one account to another are more straight forward.
This all means you can create a app on top the blockchain with any post/comment/voting mechanisms you desire and users can login with their user name and password or for more security they can give one of their derived keys that might only give you post/comment/vote permission.
I ♥ Steem!