Writability is a worthy thing to improve: GHC's -fdefer-type-errors feature is a good example. I hope Rust improves its writability in the future.
The number of tricky concepts largely do not affect writability, because you don't have to use them. The number of mandatory compile time checks do, because you have to pass them.
One way to improve writability is a mode to turn off some of checks while you are exploring. I already gave a good example from Haskell. When you are done exploring, you can turn on all the checks.
It seems like editor support would totally dominate here. That is, realtime checking of lifetimes and autocomplete/inline errors should be a much bigger improvement in write times than some way to emit invalid code.