There's still serious performance regressions.
The asset pipeline is still sort of a trainwreck.
There's still no automation for schema migrations (perhaps hobo fields will finally bring some relief for those who adopt it).
They still don't seem to have realized that code generators are a no-go.
I see zero efforts towards modernizing the bloated routing and MVC cruft to accomodate modern fat-clients (and imho the "resources"-magic was a terrible idea from the start even for regular apps).
The whole thing is still riddled with ungodly amounts of magic that likes to break in unpredictable ways (my pet bug: the monkey-patch to really suppress browser stack-traces in production changes about every 3 months and it's worrying such a bug was deliberately created in first place).
I could go on for a while... </rant>