- Lots of extra retains/releases can cause significant and unpredictable performance degradation, and even crashes: http://blog.metaobject.com/2014/06/compiler-writers-gone-wil...
- The language went from fun dynamic, optionally statically typed to strict static typing that restricts exploratory programming: http://blog.metaobject.com/2014/03/cargo-cult-typing-or-obje... http://blog.metaobject.com/2014/05/the-spidy-subset-or-avoid...
- People no longer write class-side convenience initialisers :-((
- The benefits are mostly marginal, and the non-marginal benefits are needlessly bundled with ARC
- The C interaction model of the Objective-C hybrid language was made much more complex, somewhat defeating the purpose of a hybrid language
That's off the top of my head.