Future stars who appeared in M*A*S*H episodes:
Patrick Swayze - 1981 - A couple prior roles
Teri Garr - 1973 - Many prior roles, I'm not sure how big
Joan Van Ark - 1973 - Pretty much the same as Teri Garr
John Ritter - 1973 - A few prior roles
George Wendt - 1982 - Several prior roles
Lots of others, listed here
, more or less famous than those above. Those are the most notable to me anyway.
I don't think he counts, but Leslie Nielson was in an episode in 1973, but he had been in TV & movies since 1950. This is the earliest thing I've seen him in.
Ron Howard too, but obviously he had a few notable roles by 1973, but not Happy Days, yet.