TLDR:
Hastighet, komfort, hantering, andra små klockor och visselpipor. Allt gjort av magi.
Komplett svar
Jag är säker på att det finns mer omnämnande av detta i böckerna, men här är det jag hittills hittat:
1. Hastighet
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 och Philosopher's Stone (kapitel 5)
Det jämförs med Rons gamla långsamma kvast i nästa bok:
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 och hemlighetens kammare (kapitel 4)
När Nimbus 2001 kommer ut kommer Malfoys far att få dem för hela Slytherin Quidditch.
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 och Secrets Chamber (s. 10)
2. Hantering
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 och Philosopher's Stone (kapitel 10)
I boken Quidditch Through the Ages talar det också mycket om de olika broomsticksna.
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...
Quidditch genom åldrarna (s. 1)
3. Komfort
Komfort var också en stor sak. Bromsen sträcker sig från detta:
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
Quidditch genom åldrarna (s. 1)
till de nyare modellerna:
the in vention of the Cushioning Charm by Elliot Smethwyck in 1820 went a long way towards making broomsticks more comfortable than ever before
Quidditch genom åldrarna (kapitel 9)
Massor mer
Det finns också många andra små detaljer,
endurance flying
withstanding high winds
achieve greater heights
alla som nämns i Quidditch genom åldrarna (kapitel 9)
Bonus: Mer om 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.
Quidditch genom åldrarna (kapitel 9)
Nimbus 2000 och 2001 är efterkommande av de stora broomsticksna.
Uppdatering:
Jag uppdaterar mitt svar per kommentaren nedan på hur broomsticks skiljer sig.
Kort svar: magi.
Långt svar: Häftningar på broomsticks.
Nevertheless, the first brooms bewitched for flying purposes...
Quidditch genom åldrarna (s. 1, betona min)
Det finns olika charmar gjorda för broomsticks, såsom en broms charm:
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.
Quidditch genom åldrarna (kapitel 9)
Det verkar som dessa förtrollningar kan slita av efter ålder eller försvaga i vissa fall:
... 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.
Quidditch genom åldrarna (kapitel 9)
Detta förklarar skillnaderna i besättningar, eftersom de använda charmarna varierar i kraft. Innan broomsticks massproducerades, skapade familjer sina egna, så broomsticks krafter beroende på deras ägarens kraft:
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.
Quidditch genom åldrarna (s. 1)