Jumping in a bit late here but here's my thoughts.
Probably the main sore point for me in the vanilla desktop is the text viewer. Having some basic line up/down and page up/down navigation plus simple searching ("/" style like in less) would be nice. Fixing text viewing of more than 40 columns in low res. mode so it line wraps instead of clipping would also be good. A final viewer enhancement would be to detect if the file looks like a binary format and display a hex view instead of just garbage characters. Checking a readme or other text file is a pretty common activity that I don't really want to load up 1st Word, Everest, QED, etc.
Having a built in text editor would be nice to have but not essential. Seems mostly useful when trying to bootstrap a system. As others have mentioned, a terminal emulator would be handy. Really helpful when setting up a bare system if you don't have a floppy drive on your Mac or PC. The VT52 emulator accessory looks like it's only 6kB but it would grow in size with the addition X/Y/Z-modem support plus any UI additions for port settings. Flash is over 100kB but that's much more features than needed for a basic terminal. There must be something in between out there that can be a starting point.
In any case, thanks PLM for looking working on this.