Consider:

• the novel’s brevity, the tightly focused plot, the comparatively few characters and unusually restrained dialogue and prose — all in keeping with the theme, which shapes the very style and structure of the book. Poetry compacts as much meaning and feeling into as few (aesthetically chosen) words as possible and this is the prime reason why Drood is so poetic. Poetic too in the modern sense since Dickens employs the technique, used by T S Eliot and other 20th century writers, of enriching his text with multiple literary allusions and symbols. The close ties with Macbeth and A Midsummer Night’s Dream, for example, season the book with an extraordinary mixture of supernatural horror and fairy enchantment.
• the suffocating, claustrophobic setting, so that even when the action moves to London, it’s not to open teeming streets, but to tiny rooms and an airless opium den. The novel is full of things that tightly enclose e.g. shells such as the ones from which Jasper and Edwin liberate walnuts, tombs from which Durdles frees the old ‘uns, a tavern called The Tilted [covered] Wagon and Edwin’s birthday gift to Pussy of no less than 17 pairs of gloves! Mrs Crisparkle’s mind, too — as regards the character of Neville — is closed up tight. Even the text in Drood is enclosed (in brackets) more often than in Dickens’s other novels, being fenced in twice as frequently as in A Tale of Two Cities and generally between 1.4 and 1.5 times as often as in his other books — so that if Drood had the same word-count as Dickens’s previous masterpiece, it would have about 650 bracketed items compared to Our Mutual Friend’s 450 or so.
• the many allusions to other literary works involving tyranny (usually murder) and/or close confinement e.g. ‘a poetical note of preparation’ which recalls ‘the armourers, accomplishing the knights, with busy hammers closing rivets up, give dreadful note of preparation’ from Henry V (imagine being encased from head to foot in heavy, inflexible armour); and:

‘See the old man weeps, for his fairy bride.

Oh the mistletoe bough! Oh the mistletoe bough!

At length an old chest that had long lain hid

Was found in the castle; they raised the lid,

And a skeleton form lay mouldering there,

In the bridal wreath of that lady fair.

Oh sad was her fate, in sportive jest

She hid from her lord in the old oak chest;

It closed with a spring, and her bridal bloom

Lay withering there in a living tomb.

Oh the mistletoe bough! Oh the mistletoe bough!’

(A bough is a branch and one form of death-and-resurrection god Tammuz was called the Mistletoe Branch.) The fate of the ‘fairy bride’ in Drood, of course, was to be the antithesis of the song’s one — it’s others who suffocate in Dickens’s story. the reason for Mr Tartar’s flower-boxes and small apartment when he has a large estate to move to.

• ‘The floors were scrubbed to that extent, that you might have supposed the London blacks emancipated for ever, and gone out of the land for good.’
• Mr Grewgious’s request as he prepares to leave the Nun’s House after his interview with Rosa.
• The nickname given to the head chambermaid.
• ‘This fairness troubled the Minor Canon much ... He charged against himself reproachfully that he had suppressed, so far, the two points of a second strong outbreak of temper against Edwin Drood on the part of Neville, and of the passion of jealousy having, to his own certain knowledge, flamed up in Neville's breast against him ... He was convinced of Neville's innocence of any part in the ugly disappearance; and yet ...’ And yet — moral dilemma — should he continue to repress the information or not?
• ‘You were to abolish military force, but you were first to bring all commanding officers who had done their duty, to trial by court-martial for that offence, and shoot them.’ An allusion to Governor Eyre who savagely repressed an uprising against British rule in Jamaica.
• the ‘two states of consciousness which never clash’. So that while one is brought to the fore ...
• ‘Miss Ferdinand had even surprised the company with a sprightly solo on the comb-and-curlpaper, until [she gets] ...’
• the behaviour of the two rooks at the start of Chapter 2, and the attempts to correct Mr Tope’s grammar, particularly its conclusion.
• ‘That serenely romantic state of the mind — productive for the most part of pity and forbearance — which is engendered by a sorrowful story that is all told.’ [‘sorrowful tale long past’ from ‘The Mistletoe Bough’] Dickens is alluding to catharsis of which the OED has this definition: ‘the freeing and elimination of repressed emotion’. The excerpt refers to the tranquility engendered in Minor Canon Corner but has obvious application, with his mock-fighting and strenuous exercise, to the Minor Canon himself.
• ‘This gentle and forbearing feeling of each towards the other, brought with it its reward in a softening light that seemed to shine on their position.’ Edwin and Rosa have just ended their prenatal engagement. And by openly discussing and acting upon what they’d long subconsciously wished for, they enjoy a cathartic experience. One that allows them to feel towards the other ‘pity and forbearance’. Their former ungracious behaviour was caused by a repressed desire for freedom — the strength of which desire they’d not previously been aware of.