What i love most about it is that's so annoyingly slow that it actually lets you feel how computing must have been back then.
1. I can distinctly feel the presence of the machine and a dialogue that's going on. Unlike largely transparent computer personality of the everyday use today.
2. Absence of internet distraction coupled with machine-centric environment probably resulted in a more productive developer time. That's not counting hours spent in ardent Spacewar! battles :)
"Development" would be sitting down, writing your program on sheets of paper (marked out to 80 columns so you know how many characters you get). Then either you'd key it yourself, or you'd send it off to be keyed onto cards. I'm not really familiar with the 390, but that's typically how things would work on a punched-card (i.e. batch) system.
Found a nice loophole that I could exploit. Modem users got higher priority on jobs. Plus the TRS-80 Model4P I owned was capable of terminal emulation and other nice tricks to speed up Fortran and JCL assignments. So I sat in the comfort of the dorm room and used the time I saved on EE assignments.
Much nicer being able to complete assignments within walking distance of the refrigerator. :)