I'd say that SubSpace and its predecessors were the first "haiku-competitives" -- a lineage that eventually resulted in DotA and League of Legends.
Essentially: distill gameplay to a relatively small number of rules + iterate balance until it's incredibly finely tuned + let the playerbase go nuts with whatever strategies they can come up with.
The attraction being that they're easy to pick up (few rules to learn) but incredibly deep (complex interactions and strategies).
Coupled with the fact that circa-late-90s, the idea of an MMO was still mind blowing. Games with friends and strangers?!
From memory, SubSpace also had a fairly addictive loot mechanic (tiered materials, item construction, randomly timed rare enemy spawns).