It was amazing. Multiple applications running on different servers/machines all working side-by-side on a single desktop workstation. Effectively every GUI application could automatically be run in "client-server mode" (using the terms in the traditional sense not the inverted-X11 sense) without having to write any architecture or OS specific client code.

Although technologically completely unrelated, rich browser applications also fill this niche, and even share warts like the lack of standardized UX behaviors or having issues with dealing with (subtle) difference between "client environment" implementations (different browsers or X11 "servers").

Effectively the web browser became the universal "graphical terminal" in the same way as (in the past) serial TTYs were the universal "textual terminal". Thus X11's "killer app" just slowly became irrelevant.