Statement of Regret, NT Cottesloe, London<br/>The Brothers Size, Maria Studio, Young Vic, London<br/>Desperately Seeking Susan, Novello, London

'It's textbook stuff," says Colin McFarlane's pinstriped Michael in Statement of Regret. He's talking about his long-standing but alcoholic friend and colleague, Kwaku (Don Warrington). Kwaku is obsessively mourning his father, mentally disintegrating and wrecking the black think tank he runs with Michael.

Completing the NT trilogy that began vibrantly with Elmina's Kitchen, this new play by Kwame Kwei-Armah is packed with important issues, centring on schisms within the black community and the psychological wounds left by personal bereavement and by "post-traumatic slave syndrome". Alas, though, this is not great theatre. It often feels like textbook stuff itself, stiffened with overformulated theories and phrases. At one point, the group's Oxbridge-educated intern reaches into his briefcase and pulls out Dr J DeGruy Leary's book about said syndrome. You almost hear the clunk of academic product placement.

Kwaku and his staff are seen in their plush London office. Jeremy Herrin's production looks sleek enough with its glowing computer screens and gleaming chrome chairs. Initially, we see the team enjoying a glass of fizz to celebrate the appointment of a minister for race, an idea they promoted. After that, they sit down for a meeting and that's when the rifts start – as well as the stilted exchanges.

What is meant to be a debate about their next project develops into outright conflict as the young hotshot Idrissa (Chu Omambala) argues that Kwaku's plan to demand reparations for slavery is passé. Self-flagellating white liberalism is dead, he declares, and the money for their think tank is now in more self-critical projects, such as researching why black people of African origin do better than Caribbeans in education. Idrissa himself is of African stock, as is Michael.

Kwaku, who is West Indian, condemns such projects as self-destructive, then goes bonkers and destroys the group himself. Riled by what he sees as years of Caribbean blacks being treated as inferior by their African so-called brothers, he insists on reparations for his own people. Simultaneously, he lets rip in public, with drunken racist slurs against other minorities.

Kwei-Armah is now our black British David Hare. Statement of Regret is an illuminating state-of-the-nation play about subsets of racism, as well as presenting think tanks as far from purely idealistic – here tailoring their agenda to catch the media's eye. Several of the cast gather emotional momentum, notably Clifford Samuel and Javone Prince as Kwaku's bruised offspring, and McFarlane as the patient but eventually exploding Michael. It's just a pity other members of the ensemble cannot make the dialogue – which is at worst bland – sound more sparky and natural.

Running with the theme of schisms, The Brothers Size by the fast-rising Yale graduate Tarell Alvin McCraney is a short but extremely compelling chamber piece about warring black siblings in contemporary Louisiana. Or is it, more allegorically, about one soul being riven psychologically?

Drawing on an ancient Yoruba myth, the core conflict is strong. Ogun Size (sturdy Nyasha Hatendi) is the elder sibling. A moral, hard-working car mechanic, he keeps ticking off his kid brother, Oshoosi (the looser-limbed Obi Abili) who only recently got out of jail and just wants to enjoy himself. His ex-cellmate Elegba (wiry Nathaniel Martello-White) calls Oshoosi "brother" too, but he's a sneaky devil and may be jealous. In one dream sequence, Oshoosi is literally pulled in two directions, with Ogun and Elegba as both guardian angel and vice figure.

What's remarkable about this piece is McCraney's combination of naturalism and bold stylisation. He has a great ear for slangy chat. At the same time, the characters utter their own stage directions, which is humorously intriguing, not alienating. Furthermore, director Bijan Sheibani makes the play's narrow focus riveting. With a simple chalk circle confining the playing area, he choreographs the cuts between scenes with muscular leaps at lightning speed. Hatendi is intense and Abili hilarious, expanding a karaoke rendition of "Try a Little Tenderness" into a joyous regression into teenage air-guitar.

Finally, in Desperately Seeking Susan, the bored New Jersey housewife Roberta dreams of a sexier life. Then, after smacking her head on an amnesia-inducing lamppost, she unwittingly swaps identities with the sluttishly punky Susan (played by Madonna in the Eighties movie). Well, I did toy with the idea of braining myself on my own knees during this painfully overamplified musical version. Being subjected to such substandard West End entertainment can drive a girl to desperate acts.

Ludicrously rushed and scrappy, this is sorely disappointing from Angus Jackson, who previously directed Elmina's Kitchen. The action has been shifted to 1979, apparently to add more anarchic edge. Susan's boyfriend is trying to look like Johnny Rotten, but there's something risibly tame about a pre-rehearsed chorus line, prancing around pretending to be dangerous. As for pad-ding the story with old Blondie hits, this looks like cynical commercialism. Seek something better.

'Statement of Regret' (020-7452 3000) to 10 Jan; 'The Brothers Size' (020-7922 2922) to 8 Dec; 'Desperately Seeking Susan' (0870-040 0046) to 19 April

Further viewing See Madonna and Rosanna Arquette in the 1985 film directed by Susan Seidelman. On DVD