Hi Derek: Thanks for adding your perspective. But since you don't know the other OS applications I'm thinking of - and I don't feel I can reveal their names ("to protect the guilty") - let me just propose the following hierarchy:
1) OS applications that are basically unusable w/o the "premium" version
2) OS applications that are fully usable w/o the "premium" version
3) OS applications that have enhanced "bells and whistles" in the premium version.
The ones I'm thinking of fall clearly into category #1. But Moodle does not fall into that category, IMHO. It falls into category #2. I've introduced Moodle core over the years to dozens of clients, all of whom were very happy with it (with that one exception I noted). And doesn't Moodle core meet the needs of 60+ million users? Why aren't they all screaming for ELIS, Totara, etc?
Maybe what I'm proposing here is a taxonomy for OS applications. I'd be interested in hearing whether others would validate it. Thanks.