There's a book out there that I'd like to get my hands on-- "Star Trek: The Official Guide to the Animated Series", which gives in depth information about the series.
In the meantime, I've cut and pasted a couple facts found at Wikipedia...

"Lou Scheimer of Filmation was in talks with Star Trek producer Paramount Television, TV network NBC, and creator Gene Roddenberry to create an animated spin-off series while The Original Series was still on the air, during its third season (1968–69). Paramount's director of special programming Philip Mayer and Filmation writer/animator Don R. Christensen worked together to create a proposal for a series which would target a young audience and have an educational spin. The main cast of Star Trek: The Original Series would train the teenage crew of a ship called Excalibur about space exploration; the new teenage crew included a Vulcan named Steve, an African-American boy named Bob, and a Chinese boy named Stick. However, due to the bitter relationship between Roddenberry and Paramount at the time, Scheimer was not able to get the two parties talking to each other in order to agree on a deal for several years. During this time, the project in its original form was phased out.
"A deal was finally reached in early 1973, and publicly announced in early March 1973. Because of NBC's strong interest in the series, Roddenberry and Filmation were allotted very generous terms: a guaranteed minimum of two seasons with a combined 22 episodes, a budget of $75,000 per episode, and full creative control in Gene Roddenberry's hands."

"Roddenberry and Filmation agreed that the series should be for all ages, rather than the children-oriented approach of the original proposal, and at Roddenberry's suggestion The Original Series script editor D. C. Fontana was hired as the series' story editor and associate producer. Despite the meager payment for writers ($1,300 per script, with no residuals), the opportunity to write a Star Trek episode without the special effects limitations of live action proved appealing, and many of The Original Series writers joined the staff. Fontana steered the series away from the romantic and sexual elements of The Original Series, as she felt children would not be interested in them and she wanted The Animated Series to appeal to children as well as adults."

The Animated Series featured most of the original cast voicing their characters. The major exception was the character of Pavel Chekov (Walter Koenig), who did not appear in the series because the series' budget could not afford the complete cast. He was replaced by Lieutenant Arex, whose Edosian species had three arms and three legs; Lieutenant M'Ress, a female Caitian, sometimes stood in for Uhura as communications officer. Besides performing their characters Montgomery Scott and Christine Chapel, James Doohan and Majel Barrett also performed the voices of Arex and M'Ress, respectively. Barrett also voiced various other characters and the main computer.

Initially, Filmation was only going to use the voices of William Shatner, Leonard Nimoy, DeForest Kelley, Doohan and Barrett. Doohan and Barrett would also perform the voices of Sulu and Uhura. Nimoy refused to voice Spock in the series unless Nichelle Nichols and George Takei were added to the cast, claiming that Sulu and Uhura were proof of the ethnic diversity of the 23rd century and should not be recast. Nimoy also took this stand as a matter of principle, as he knew of the financial troubles many of his Star Trek co-stars were facing after cancellation of the series. According to Scheimer, when Nimoy pointed out that the casting would cut the only two minority actors from the series, "We were horrified at our unintended slight, made all the worse because we were the one studio who had been championing diversity in its output." Koenig was not forgotten, as Filmation were able to assuage Nimoy's complaints on his account by buying a script from Koenig for one episode ("The Infinite Vulcan").

Voice recording began in June 1973, with the first three episodes recorded as an ensemble, i.e. all the voice actors for the episode in the room at the same time.] Later episodes used the more typical model of recording the voice actors separately to work around their other commitments. William Shatner and Leonard Nimoy, who were both touring in plays at the time, recorded their lines in whatever city they happened to be performing in and had the tapes shipped to the studio.

Interesting.

Mind you, I pretty much knew all this beforehand.

Well, it's a good thing the book is longer than 5 paragraphs!

Yup.