You might think turntables have an easy job: just spin the platter supporting the record. Hold on, spinning at exactly thirty-three and a third revolutions per minute, without the slightest variance and flutter is a surprisingly difficult task to pull off.
Remember, too that the phono cartridge's stylus tracing the LP's groove is a remarkably sensitive device; it "reads" groove wiggles that can be smaller than a wavelength of light. But the stylus tracing the groove can't distinguish between groove wiggles and other vibrations, such as those from the turntable's extraneous motor noise, or the sound coming out of the speakers in the room with the turntable. The bearing the platter rests upon, and the tonearm's bearings also make noise, which are also picked up by the stylus.
A perfect turntable's platter would spin at the exact right speed; its motor and bearings would produce absolutely no noise, and the turntable/platter system would be completely isolated from its environment. No such turntable exists, but high-end turntables get a lot closer to that ideal than budget contenders.
That's why the very best turntables seem quite a bit quieter than lesser turntables; they produce less rumble and groove noise, and clicks and pops seem less intrusive. Cheap, poorly designed turntables exacerbate groove noise and tend to sound screechy. Most budget 'tables have limited bass power and poor bass definition.
Audiophiles know LPs sound vastly better when played on a great turntable. Sure, the quality of the phono cartridge can play a very significant role, but a great cartridge is great mostly because it more precisely decodes the groove wiggles than a lesser cartridge would. If the turntable adds rumble, motor, and bearing noise, and picks up sound from the speakers, the great cartridge can't sound all that great. Conversely, if you put an average phono cartridge on an audiophile grade turntable, the cartridge would decode more of the music pressed into the grooves, with less background sound, so the music would sound better. A lot better.
In the real world, turntables really do sound very different from each other. The differences aren't subtle, and Jeff Dorgay, over at ToneAudio magazine, was mightily impressed with the Rega RP1 turntable's sound. Dorgay thinks it's one of the best entry-level audiophile turntables you can buy.