Upvoted for a number of interesting and unconventional opinions.
Rolling your own JSON, XML, etc parsers raises some eyebrows. I guess it's sort of okay if you expect to only be reading your local config files versus arbitrary web content. Maybe then it's okay to trade off raw speed for not handing a ton of formatting and syntax edge cases.
Writing GUI apps against the native framework. Yup, it'll be a major pain to ever port to another platform. But if you don't care to do that, nothing's as fast and clean both to develop and for the end-user to use as every platform's native framework.
Unit tests are sometimes overrated. IMO, they're much more suited to building apps in dynamic languages. Unit tests are a lot less important if you have a good type system and compiler checking things already. I've heard it described before that half of the benefit of unit testing in these languages is forcing yourself to structure code in testable modules with clean interfaces. But if you can just do that anyways, the actual tests are less important.