A former U.S. official who dealt with the Middle East said the strike was especially notable because it targeted the leader of a state apparatus, as opposed to a non-state actor.
“We need to be prepared that we’re now at war,” he said.
This isn't like your typical drone strike on a militant terrorist. It's roughly the equivalent of Iran killing the head of the CIA or of a branch of the US Military or someone else with the full backing of a major state behind him.
Traditionally countries have not engaged in assassination of leadership (with the notable exception of North Korea). It's a dramatic escalation and hard to see how it doesn't lead to another ground war in the Middle East.
The Saudis will be happy though..