You can have at least 2 types of 2nd layer platforms built on top of steem:

- A subchain (that can be a smart contract platform) that has it's own peer to peer consensus rules that only includes operations that are considered irreversable on steem. Such a chain would be slower by nature but the advantage is that any bugs or forks would not have any negative effects on the the main chain.

- A second layer where everyone that uses it agrees that if an operation follows certain rules it is considered valid but...there is no peer to peer consensus mechanism built in. Steemnonsters is an example of this type of solution.

The later is not much different than a private transaction conducted between two parties. It's no different than paying someone for a product or service using steem (or any other crypto) but if they don't deliver I have no way of enforcing it on chain (although with steem you could use the escrow function to delay payment until the product or service is delivered).

You could even build some very fancy apps using a combination of escrow, multisig and the hierarchical nature of the key structure of steem along with the secondary interpretation of data that "soft" consensus brings.