I haven't replied to this before because I couldn't summarize what I like. I agree with everybody's reasons.
I think my favorite part is the variety. In my oldest solo game, my Serene Scientists, the Tictsin, have Partnerships with three races, and have spread all over the map while holding their own in a protracted war with the Xi-Chung. In my first PBW game, my Honorable Warriors, the Rrurrr, fight on to certain death rather than surrender to a technologically superior alliance. (Hi, Askan & Spoo)
In my other PBW game, my Psychotic Engineers, the Mi-Go, have cultivated numerous alliances as they prepare for their next wave of expansion.
I enjoy writing in-character diplomacy with human players, too. The Rrurrr often speak of Honor. the Mi-Go have buzzing rrazpy whizperring voicez...they do not uze capital letterrz...prroperr namez excepted...no conjunctionz...theirr only punctuation iz the ellipziz...
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Cap'n Q
The most merciful thing in the world, I think, is the inability of the
human mind to correlate all of its contents. We live on a placid
island of ignorance in the midst of black seas of infinity, and it was
not meant that we should go far. -- HP Lovecraft, "The Call of Cthulhu"