Programming in the REPL is something I do all the time and generally enjoy, but I sometimes feel like the actual design of the program then becomes too focused around REPL usage. Too much abstraction can harm performance and waste time. The REPL encourages this by making you think about the internals of your program as an interface that will actually get used, when in reality some function may get called twice ever from within your own code, and isn't really meant to be exposed. But because you are repeatedly invoking it manually via the REPL, it feels like an interface worthy of considerable design.

Maybe it is, and maybe it isn't. I've just noticed this tendency in myself when working in REPLs. Not every part of your program is an API.