So why does it work so well? A lot of it has to do with Jane Campion's direction. She has a great skill in making period detail convincing. Dimly lit rooms, scuffed furniture and fraying bonnets go a long way to recreating a convincing early 19th century milieu. The script is completely immersive - the dialogue feels casual and improvised, and while there seem to be a few verbal anachronisms this is preferable to the over-researched, stilted dialogue that's the downfall of the standard BBC costume drama. It's also full of pauses and silences - Campion has clearly absorbed Keats' idea of 'negative capability', and often leaves us to absorb without mediation the rustling of leaves, distant birdsong or even complete silence.
There's a clear feminism at work here which also helps - this isn't the story of John Keats' love affair with Fanny Brawne, it's the story of Fanny Brawne's love affair with John Keats, and the film insists on keeping Brawne's inner life at the heart of proceedings. This means Abby Cornish as Brawne has a lot to carry, and she's well up to the task, depicting a Brawne as a steely, if brittle young socialite who finds in Keats the outlet for an emotional honesty that's exhilarating and ultimately disastrous. Her gradual collapse and final breakdown as her lover is first whisked away to Italy and his inevitable death is absolutely convincing.
Her supporting cast is excellent. Ben Whishaw is miles away from the tortured artist stereotype - his Keats is low-key and sardonic, although he recites his own poetry beautifully ('Ode to a Nightingale' is performed in its entirety over the closing credits. Nobody left.). Despite what you've heard, Paul Schneider's Scottish accent is fine, and he creates sympathy for what could have been quite an unpleasant character..
You need to open yourself up to a film like this - a moment of cynicism and the spell will be broken. But if you're prepared, like Keats, to let it wash over you, you'll be amply rewarded.