I would get a copy of the book Film Developing Cookbook by S. Anchell and read the first chapter which describes the different categories of film developers. Then I would try a developer from each category and compare the results with the others.
I use a fair amount of HC-110, but to my taste it isn't "here's a developer that does something special", it's "here's a convenient all-round developer that does everything decently". I'm not sure it would be all that exciting for someone who's basically happy with the results and workflow from D-76 but looking at Exploring Something Different.
Diafine is a different kettle of fish. I think most people come to it looking for speed gains, and while opinions differ on how real those are, it certainly has really distinctive looks with certain films (especially Tri-X) and the compensation benefits of a two-bath developer. I've come around to where I use it mainly as a strong compensator for high-contrast situations like night shooting, and for specific cases like IR film (it tames the Wood effect a bit, making the images a little less unnatural-looking). It seems worth playing with as something that may open up different possibilities---I don't think very many people use it as their "standard" developer nowadays.
All IMHO, obviously, and my opinion is likely worth what you paid for it. :-)
San Diego, CA, USA
The lady of the house has to be a pretty swell sort of person to put up with the annoyance of a photographer.
-The Little Technical Library, _Developing, Printing, And Enlarging_
How would you like to change your negatives? (what's lacking?)
Nothing's lacking, per se. I just want to have the experience of a few key developers under my belt so that I can compare the workflow and results and have more insight into what developer might be up to a particular look or task.
I'm thinking Rodinal at the moment, but I'm sensing a lot of love for HC-110 and Pyro.