Velocidade, conforto, manuseio, outros pequenos sinos e assobios. Tudo feito por magia.
Resposta completa
Tenho certeza de que há mais menções sobre isso nos livros, mas aqui está o que eu encontrei até agora:
1. Velocidade
Several boys of about Harry's age had their noses pressed against a window with broomsticks in it. "Look," Harry heard one of them say, "the new Nimbus Two Thousand -- fastest ever --"
Harry Potter e a Pedra Filosofal (cap. 5)
É comparado com a velha vassoura lenta de Ron no próximo livro:
They took turns riding Harry's Nimbus Two Thousand, which was easily the best broom; Ron's old Shooting Star was often outstripped by passing butterflies.
Harry Potter e a Câmara Secreta (cap. 4)
Então, quando o Nimbus 2001 é lançado, o pai de Malfoy os recebe para o quadribol da Sonserina.
He was nervous, mainly at the thought of what Wood would say if Gryffindor lost, but also at the idea of facing a team mounted on the fastest racing brooms gold could buy.
[a few pages later]
"All right there, Scarhead?" yelled Malfoy, shooting underneath him as though to show off the speed of his broom.
Harry Potter e a Câmara Secreta (cap. 10)
2. Manipulação
Too eager to fly again to wait for Wood, Harry mounted his broomstick and kicked off from the ground. What a feeling -- he swooped in and out of the goal posts and then sped up and down the field. The Nimbus Two Thousand turned wherever he wanted at his lightest touch.
Harry Potter e a Pedra Filosofal (cap. 10)
Além disso, no livro Quadribol através dos tempos, fala muito sobre as diferentes vassouras.
As wizarding families in those days made their own brooms, there was enormous variation in the speed, comfort and handling of the transport available to them.
...
... broomsticks were sufficiently advanced to allow fliers to turn corners and vary their speed and height...
Quadribol através dos tempos (cap. 1)
3. Conforto
O conforto também foi um grande negócio. As vassouras variam disso:
A German illuminated manuscript of this period shows three warlocks dismounting from their brooms with looks of exquisite discomfort on their faces. Guthrie Lochrin, a Scottish wizard writing in 1107, spoke of the “splinter- filled buttocks and bulging piles” he suffered after a short broom ride from Montrose to Arbroath
Quadribol através dos tempos (cap. 1)
para os modelos mais novos:
the in vention of the Cushioning Charm by Elliot Smethwyck in 1820 went a long way towards making broomsticks more comfortable than ever before
Quadribol através dos tempos (cap. 9)
Muito mais
Existem também muitos outros pequenos detalhes,
endurance flying
withstanding high winds
achieve greater heights
todos mencionados em Quadribol Através das Eras (cap. 9)
Bônus: Mais sobre o Nimbus
In 1967 the broom world was galvanised by the formation of the Nimbus Racing Broom Company. Nothing like the Nimbus 1000 had ever been seen before. Reaching speeds of up to a hundred miles per hour, capable of turning 360 degrees at a fixed point in mid-air, the Nimbus combined the reliability of the old Oakshaft 79 with the easy handling of the best Cleansweeps. The Nimbus immediately became the broom preferred by professional Quidditch teams across Europe, and the subsequent models (1001, 1500, and 1700) have kept the Nimbus Racing Broom Company at the top of the field.
Quadribol através dos tempos (cap. 9)
Os Nimbus 2000 e 2001 são descendentes dessas grandes vassouras.
Atualização:
Estou atualizando minha resposta com o comentário abaixo, em como as vassouras diferem.
Resposta curta: mágica.
Resposta longa: enfeitiçamentos nas vassouras.
Nevertheless, the first brooms bewitched for flying purposes...
Quadribol através dos tempos (capítulo 1, ênfase minha)
Existem diferentes feitiços feitos para vassouras, como um feitiço de freio:
The patented Horton–Keitch braking charm meant that 50 Quidditch players were much less likely to overshoot goals or fly offside, and the Comet now became the broom of preference for many British and Irish teams in consequence.
Quadribol através dos tempos (cap. 9)
Parece que esses encantamentos podem se desgastar depois da idade ou enfraquecer em alguns casos:
... the Swiftstick nevertheless has a tendency to lose power in ascent...
Unfortunately, after its initial burst of popularity, the Shooting Star was found to lose speed and height as it aged, and Universal Brooms went out of business in 1978.
... the Twigger has been found to warp under high speeds and has gained the unlucky reputation of being flown by wizards with more Galleons than sense.
Quadribol através dos tempos (cap. 9)
Isso explica as diferenças nas vassouras, já que os feitiços usados variam em potência. Antes que as vassouras fossem produzidas em massa, as famílias se apropriaram, então os poderes dos cabos de vassoura dependem do poder de seus donos:
As wizarding families in those days made their own brooms, there was enormous variation in the speed comfort, and handling of the transport available to them.
Quadribol através dos tempos (cap. 1)