Interface Improvements for Mod Support
This is a great app, it's simple and does what it needs to do but could use some improvement. the main interface should have 3 panels instead of 2. There should be panel on the left that has a list of all saved .zdl profiles. Clicking on one instantly loads it into zdl and shows all of the mods in the "External Files" panel and selects the source ports/Iwad. ("Extra Command Line Arguments" would also be loaded) It would make more logical sense to have the panels arranged from left to right: .zdl profiles -> Source Port/IWADS -> External Files.
Having a default location for loading/saving .zdl files would be another good QOL feature. (It would also be required for the profile panel)
Bonus points for allowing custom icons and/or screenshots for each profile.
Agreed; being able to export setups is nice (and absolutely essential for allowing mod creators to ship importable files to make setup a breeze), but for day-to-day usage I would much rather have an "internal" list of mod setups than manage *.ZDL files at the OS level.
Seems to me that making ZDL "aware" of the concept of a list of persistent mod profiles could also pave the way for stuff like automatic mod updates in a future release; each mod has a URI in its metadata that contains an update link which ZDL could iterate through to...well, we're getting way out of scope for this issue but you get the idea. :)
It would be nice if you were able to change load (files) orders, like this

While we are on this topic, is it possible to add ability to automatically load .deh files alongside specified WADs?
I was rereading the top post, and it sounds like it would take the model of a music library manager (e.g iTunes) and treat the .zdl files as playlists.
To me, if we're using a music library manager metaphor, the .zdl files are more like individual songs. Basically, the idea is to replace having a directory full of .zdl files with a list view inside ZDL (maybe with columns to show metadata?).