What does a tonearm contribute to the sound of a turntable?

@simao 

"So those of you with the ethos on this: I have the Jelco 750EB coupled with a Hana SL cartridge. Compliance complementary?"

It is borderline. Light cartridge and medium compliance. 

The Jelco 750 series, however, offer quite a bit of flexibility due to the
fluid damping. I use an even more compliant cartridge (an Accuphase AC2)
on my 750D using the fluid damping and it sounds very good. 

I would definitely use the fluid damping on your Jelco if you are not
already.   

Fluid damping causes all kinds of problems of it's own. The solution is worse than the cure. 
I went from various iterations of the original SME 3009 arm to various VPI arms including two of their 3D arms to my current two arms, Reed 3P's. 
I have had 9", 10.5, and 12. Steel, alloy, carbon, plastic (VPI 3D, call it "composite" if you like, it's junk IMHO), and now one Reed is 12" cocobolo and one is 10.5 in ebony. 
Just my opinion and experience-everything about the arm makes a difference. How it is tightened, how the vertical and horizontal motion is designed, the materials, the length and materials used for the armwand, shell, clips, wiring, hell, even the mounting and armboard materials have an effect on overall sound. 
Many of these things are not worth sweating about and some are. But they all have an effect on the overall sound. 
I use the same old analogy often but it is a good one. A pro can take a better photo with his phone than you can with a 10K Nikon set up. But that doesn't mean that the 10K Nikon set up is not worth the money in the right hands. Likewise, you can get great sound from a modest set-up if you set it up optimally. But.....

@hdm Thanks for the feedback. I have indeed damped the tonearm according to its specs, starting with two drops in the well so it's just under halfway.

@simao look at the Jelco 850. Big upgrade.

Larry needs to gain some experience  and knowledge before comparing a high end vinyl rig and quality acetate to the sound of a telephone.  That’s like comparing Lawrence Olivier to Popeye, or Raquel Welch to Phyllis Diller.