I'm still thinking about your question: "So, fellow Audiogoners, what do you think has the greatest effect on vinyl playback as far as only the turntable itself, and what do you think can be done to ammeliorate those effects."
I think of the turntable as a system, and in a system everything interacts with everything else to a greater or lesser degree. Maybe Atmasphere's answer is the best, since a mat might be a general solution applicable across many systems, yet there are clearly some turntables that might perform better without a mat, so maybe not?
One of the great things about this hobby is the fine variety of solutions developed over the years which represent totally different systems for playing back vinyl. The fact that outstanding results can be obtained with extremely different solutions leads me to think that your question cannot be answered generically: it will depend upon which system you are using.
I'm currently building a turntable and experimenting with materials and their acoustic properties. There's a lot of misinformation and some good information on the web (check out AudioQualia for an example with lot of both), but the one thing that I've learned is that results of combinations of materials can be non-intuitive in terms of their sonic results. For instance, the base plate that supports my main bearing is a piece of aluminum that is 1"thick. If I hang it on a string and tap it I get a huge "Riiiing." Not such a good thing, since some audio frequencies will resonate with that and it affects the sound signature. But if I sandwich the aluminum with a similar mass of 3/8" steel, I can almost completely cancel these resonant frequencies. But if I use MDF in the sandwich instead, I get some new resonances, and lower the frequency at which it rings. It completely changes the sound signature of the turntable without making any other changes. And that is just the plinth! Consider similar materials considerations for the platter and for the base/feet/platform, etc. And of course the decisions made about these things affect the above results with the plinth as they interact.
And then there's the motor. I've got a nice 3-phase motor with a solid-state 3-module T-amp based regulator, where I can adjust the voltage and frequency to minimize motor noise and maximize stability. The motor controller makes a HUGE impact on the perceived "drive" of the music. But in another design, with a DC motor, or an idler drive, maybe this would not matter so much. And so on, and so on for each component part of the turntable.
So congratulations, I think you've asked an unanswerable question. I think in the context of a specific system, there might be some answers, but mostly I think they cannot be generalized to other systems. And the general answers that can be provided are too non-specific to be helpful to you in making upgrade decisions. They all have caveats based on the system in which they are applied. I'd love to hear some general answers that ARE universal, but I'm pretty certain every one will have situations where they don't work.
Ain't this stuff fun?
