Here Hollywood makes a film about Hollywood remaking a Hollywood TV series. The outermost frame is unspoken (this isn't a documentary about "the making of") but that just heightens the irony of actors making fun of, and celebrating, acting (including bad acting) and its made-for-the-camera cultural milieu.
For Nicole Kidman to frame herself as an actress playing an actress, she almost has to come from another dimension -- which she somewhat manages to do, as a witch who knows her own nothing-to-sneeze-at charms. Ferrell is charming too, and of course more obnoxious as Kidman's male foil.
Shirley Maclaine and Michael Caine play the older parallel couple (as Endora and Nigel), and covertly bewitch one another, likewise to foment romance. So is love authentic if it's so artificially begotten? The movie industry is forever asking itself this question, given the impersonal machinery of love-making is so in-your-face all the time (smile for the camera now).
I don't remember the TV show well enough to recall Uncle Arthur, originally played by Paul Lynde. His appearance in the person of Steve Carrell smashes the illusion: he starts in Wyatt's dream sequence, but is still there when he wakes up -- the layered realities collapse into one (hi Steve). Nor did I recall that Darrin got recast some distance into the original series, a fact Farrell's Wyatt alludes to, when feeling nervous about his career.
The film stays light-hearted throughout, faithful to the spirit of 1960s USA sitcoms and their suburban Pleasantville fluffiness -- is a traditional mid-summer comedy in other words.