Full Idea
Einstein's first theory is 'special' because it only deals with observers who move in a straight line at constant speeds with respect to one another.
Gist of Idea
The theory is 'special' because it sticks to observers moving straight, at constant speeds
Source
report of Albert Einstein (works [1915]) by Graham Farmelo - The Strangest Man 03
Book Reference
Farmelo,Graham: 'The Strangest Man [Paul Dirac]' [Faber 2009], p.40
A Reaction
Most theories of this period seem to have focused on the simplest cases, for obvious reasons.