My question is, what is the barrier to entry?

It's still unclear. In the initial version of the white paper there are mentions of either 1SBD or 1000SBD as the required fees to launch your own SMT. We'll see when it launches.

it seems we could do this anyway with ETH?

No, you can't. Suppose you write the entire application layer yourself (code for posting, voting, curation, vesting mechanisms, etc) on Ethereum you will still have to pay gas for each operation. What you get with SMT is Steemit-on-demand.