While my earlier comment here relates to this, I think it is very much possible to start building Dapps or smart contracts on Steem. But I don't think there will be one dominant platform where all Dapps will reside, I see future where cross blockchain projects or contracts come to life, for example between Ethereum to Bitshares or Steem to Ethereum, etc. It certainly opens up a lot of doors.
I think most of the DAPPS will go on Ethereum Blockchain b/c that's what it is designed to do. For Social Media Steemit is the place. The Steemit dollar peg is very interesting for remittance and store of value. Its too early to tell what we happen. One thing for sure no Steemit is booming with life, considering its just a month old!
Steem has different infrastructure but it is faster than Ethereum, which is key component for Dapps.
Well, same goes for eth, not mature enough. I do agree though, it promised to be or designed to be for dapps but proving that, is another story, the best thing happened in Ethereum is DAO hack.