Cogging exists, but it isn't always noticeable unless there is another turntable in the room that doesn't cog. It sounds a lot like digital jitter from a cheap CD player, if the turntable is a truly inferior one. More often, such comparisons are made with much better turntables, of course. By that, I mean A-B comparisons. In those cases, the apparent symptom is a very subtle smearing of the mids and highs that presents itself like a veil over the music. In a large symphonic work, background instruments are more likely to be diminished, where their presence is more prominent from a turntable that adequately addresses the issue. Also, dynamics are affected. Take away the better turntable, and the smearing turntable becomes acceptable again to most people after a few sides are played. We are talking about subtleties, after all. Still, it is an unacceptable condition for the guy who spent a lot of money on analog playback. Maybe he doesn't know it yet, as that seems to be the case most of the time.
In my experience, the turntables that cog most are those which do not regenerate power, but employ a feedback scheme that corrects often. Bad motors cog, too. High-end turntables aren't impervious to bad motor choices by their designers, unfortunately. I would surmise that motor selection is one of the more difficult aspects of turntable design, and you guys would be surprised at who in the industry takes the cheap path with the motor.
