[quote="foobar":2pc0qear]It seems like the main Doom ports are Doom Legacy, Jdoom, PRboom, and Vavoom. Development of Doom Legacy and PRboom seems to have stalled.
I think it depends much on which preference you have for a Doom port, my opinion of the main Doom ports is Boom, ZDoom, Vavoom, Doomsday (JDoom, JHeretic /& JHexen). All other ports aren't that good (again, it's my personal opinion).
[quote="foobar":2pc0qear]Does anyone know the relative strengths/merits/weaknesses of the various ports?
Every port is based on the same sources (Doom, Heretic, Hexen, and some ports even recreate the Strife source), so all of them are based on the same stuff. As for merits and strengths, it's another story, it's really difficult to take a firm opinion on what is a merit or weakness, as every Doom port has it's own features and enhancements (some of them are available to various ports, lik ACS support, and all the Doom engine based games support).
[quote="foobar":2pc0qear]In what ways are the design goals different between these ports, and if the design goals are not different, why haven't the various ports merged?
They differ a lot from each one, there are source ports based on the idea of making the game really customizable, without the need of modifying the source code (ZDoom and Vavoom enter this category, I would say Edge and others do this too), there are other ports that are based on the idea of making Doom look really good by using new video technologies and stuff like that (Doomsday does this), all their features are based on making this possible, and there are a lot of other ports that don't have a clear idea of what to do, just to make the author ideas possible.
I think that merging the work of all the different authors in a single port would be extremely difficult, as every different port works the same features in different way, depending much on the way the source code is made, ZDoom it's far away from what the original source code from Doom was, and other ports have managed to merge Quake engine code into the Doom engine (like Vavoom) to create new ways of building maps (real 3d architecture), which is way cool!
I think the selection of a Doom port mainly goes to what you want to do or see in it and your preference of it.