I know that in the original run, the blanks of Rhoda's departure where filled in when Rhoda premiered.
However, I wish there was more said about why Rhoda left on MTM. You keep expecting Rhoda to walk in Mary's apartment.
I noticed in a few early 5th season episodes, there were a few gatherings in Mary's apartment to compensate Rhoda being gone.

I always think they were seeing on MTMS how well the spinoff Rhoda did before mentioning Rhoda was back in New York.

Once the wedding plans are under way,Rhoda is mentioned and the wedding on MTMS.

Rhoda's reason for leaving is all explained in the Rhoda pilot,sadly the Mary Tyler Moore scene was cut from the DVD of the pilot,here it is though:

http://youtu.be/Ps1_IH4TOK0