The scripts were were well written, the plotting well thought out and the characters - and ensemble cast - excellent. In particular, congratulations to Mark Ryan - who became Nasir the Saracen. Due to be killed off at the end of the the first episode, he was so good, that he remained as a permanent cast member.
Ultimately, though, the series' immediate appeal was due to the amazing on-screen chemistry between its two leads, Michael Praed and the beautiful Judi Trott as (the first) Robin and his Marion. Praed's youthful, dark, good looks had an almost 'fey' quality which made the magical element entirely believable. (And by 'fey' I don't mean to imply any loss of masculinity.) The first two seasons - with Praed - were by far the best.
In the third season, Jason Connery had a hell of a task following an actor so perfect for the Robin role, but he did reasonably well and the strong ensemble cast carried the change of lead well. Richard Carpeneter's wise decision to make the new 'Robin Hood' a completely different character with a completely different background was a very wise move.
I suppose any review should mention the immense debt Costner's Robin Hood Prince of Thieves owes to Robin of Sherwood. Some might say that 'debt' was putting it mildly! Costner not only retained the Saracen (who until Nasir had never been a Robin Hood character at all) but he filmed in many of the same locations; used the same horsemaster (Stevie Dent) and clung to the magical elements - though being a good old American boy stuck to the idea of Christianity good - old religion bad, whereas Robin of Sherwood often showed the political corruption of the Church (historically accurate)and the simple spirituality of the old religion (unrelated to 'black' magic). Alan Rickman's OTT sheriff was a wilder version of Nickolas Grace's sly characterisation.
What more can I say? If you've never seen Robin of Sherwood, rush out and buy the complete thing on DVD - I guarantee you'll watch it again and again for the lovely filmic quality of the camera work, the leisurely but never dull pacing (which invests in the attention span and intelligence of its audience), the acting, the ideas, the very real emotional kick and (much praised) the delightful music by Clannad.
And did I mention Michael Pread...?