They don't use ASCII, they use EBCDIC, which has its elegant parts and its horrors (English letters are not contiguous in code-space).
The screen isn't treated as an infinite-scroll dot matrix printer, it's treated as a... as a very old visual text editor, with stencil masks, I guess.
The UI is very unified by alien conventions.
Files are not files, they are record stores.
There's a weird batch language that you use for everything.
If you're not writing a batch program, you'll be writing on top of CICS, which is a framework for, uh, realtime message processing programs and GUIs.
The DBs have their own idiosyncrasies. Etc' etc.