John Raitt was a great musical-theatre singer. Emphasis on singer. He could act, but he wasn't primarily trained as an actor, and Rodgers and Hammerstein didn't pick him for Carousel for acting chops, but for vocal excellence. (as Ethan Mordden has pointed out, Rodgers always wanted excellent singing voices, even if they belonged to relatively inexperienced actor). His specialty was delivering a great show tune in a big, rich, operatically-trained baritone voice. When we read that he played the lead in Rossini's Barber of Seville before R&H picked him for the Chicago company of Oklahoma! it's a reminder of the fact that in the '40s there wasn't much of a dividing line between musical-theatre singing and classical singing; Broadway musicals abounded in well-produced, well-trained voices that didn't need microphones to be heard in the back of the theatre. Now, of course, what we think of as Broadway singing is almost entirely microphone-directed, and "trained" singers in musical theatre are singers who were specifically trained to do that kind of microphone singing. But for the music of Rodgers and Hammerstein, classical training is far more valuable and appropriate than what passes for "musical theatre training" among many singing coaches.
Raitt was never quite a big star; after Carousel, the only hit show he starred in (in the original run, anyway) was The Pajama Game, and he wasn't the first choice for that: he came in to replace Ralph Meeker, who was fired because he couldn't sing. By 1954 musicals were more heavily emphasizing other things besides singing; the rest of the Pajama Game cast consisted of people who were primarily actors (Janis Paige) or dancers (Carol Haney) and could more or less sing. Raitt, like William Johnson (a great Broadway baritone who died young), was first and foremost a singer, and therefore not the first choice for many roles in a time when more and more non-singers were populating the casts of Broadway musicals.
Raitt is sometimes compared with the guy who created the lead in R&H's Oklahoma! Alfred Drake. But the comparison doesn't quite hold. For one thing, Drake really was a big, name-above-the-title star, which Raitt never really was. For another thing, Drake was kind of an ironic, self-aware performer, a specialist in parody (his singing was a spoof of schmaltzy operetta singing, his vehicles, like Kean and Kismet, were at least partly satirical). Raitt was a performer without apparent irony, without distance between himself and the audience or himself and the material. He symbolized the musical-theatre trouper, the guy who didn't think this stuff was corny. If you went in thinking these songs were corny, his sincerity won you over; for the larger portion of the audience that saw nothing corny about Kansas in August, it was great to hear a first-rate singer who didn't condescend to the material.
Mark Evanier has a good post on Raitt and his willingness to give of his best in any venue.