(Yes, it works as action, not as drama or art-house. If you want slow existential agony or impressionistic vignettes, go elsewhere.)
It can be summed up with one word: 'balance'.
Whedon - the wunderkind behind Buffy and Firefly - clearly knows how to deliver action. The Avengers, as with the other outings in Marvel's comic franchise (Iron Man, The Hulk, Thor, Captain America), is a beautiful balance of CGI artistry with old-fashioned bodily performance.
All the leads obviously trained exhaustively to fight, jump, fall, roll and generally look physical. It looks palpable. Their performances blend beautifully with the CGI, which amplifies their physicality, and provides them with fantastic enemies and weapons. Take, for example, the fight between Thor (Chris Hemsworth) and Iron Man (Robert Downey Junior).
This is only a small sample (and not the best), but it makes the point. Avengers is full of these spectacular match-ups, which give each hero (and villain) the chance to show off their characteristic powers.
Which brings me to the next balance: between action and character. While Avengers is a visceral entertainment, it also has genuine characters. This is partly because the previous Marvel films have given each hero (with the exception of Hawkeye and Black WIdow) a good backstory.
But it is also because Whedon regularly stops the action, and gets characters talking. The dialogue is occasionally clunky, but overall it is sharp, funny and - most importantly - characteristic.
For example, Chris Evans' Captain America is brave and earnest, but also well aware of his limitations: as an 'enhanced' soldier, but also as a man out of his era. He says little, but says it with blunt sincerity. Mark Ruffalo's Bruce Banner is warm, softly-spoken, but a tone of threat always hums as he talks: his Hulk is never far away. Tom Hiddleston's villainous Loki is at turns sweet, cold and batshit crazy which suits his damaged persona perfectly.
Whedon's strength in this film is to take these characters, put them in a room, and get them talking - and letting good performers shine. Watch silver-tongued Downey Junior and eloquently menacing Hiddleston in this scene:
As Downey Junior's performance suggests, there is one more balance to recognise in Avengers: between dramatic conflict and humour. There is plenty of comic-book pathos in this film - and very little bathos, which is nice - but it is also funny.
As with Firefly and Buffy, Whedon knows when to stop the roller-coaster and do a little slapstick, situational gag or stand-up. In the screening I saw, folks laughed out loud. Together. And kept laughing. It's not just Downey Junior - although he's in top form. It's also Hemsworth's Thor, Ruffalo's Banner, Samuel L. Jackson's charismatic Nick Fury. And Hulk: a revelation of CGI fun.
(I won't give away the jokes, but two jokes involving Hulk had my eyes wet with laughter.)
Yes, the plot of Avengers is absurd at times. But, as with J.J. Abrams' Star Trek, it does not matter. The film delivers such action, character and laughter that the plot holes are filled with the smiling stop-gap of great entertainment. As Gerard Wood from Science Fiction World put it: "absofrakkinglutely awesome."
Action fans: assemble!