Annamalai nailed it. I've been listening to Carnatic music for almost 30 years and nobody anticipates and shadows the main artist throughout a 3 or 4 hour concert like VR does. There are many mridangam vidwans who excel at kanakku and racy patterns - some of them even provide periods of faithful accompaniment, but, none of them (PMI, PSP, TU, UKS, PRR, TS included) do the faithful shadowing - without ever missing a step - all through a concert.
One of the oldest MMI tapes that I can recollect listening to was the one with Sarasasamadana, Saarasamukhi, MaaJanaki and Eppo Varuvaro. I would listen to the tape - especially MaaJanaki - twice - once to focus on MMI's kalpanaswaras and the second time, to focus on VR's mridangam.
Another difference that I have observed during all the live concerts that I've attended - Mridangam artists usually devote about 60% of their attention to the main artist and the violinist; the rest of their attention is devoted to looking out at the audience. The only exception I have seen is VR. He focuses with an almost obsessive intensity on the main artist and violinist - and almost never ever looks away - especially when he is actively playing.
To a lay listener, listening to a VR accompanied concert gives the impression that the performance has been carefully orchestrated and rehearsed - such is his perfection and consistency.
Annamalai mentioned VR accompanying MMI, MDR and AB. Another inimitable and simply spellbinding performance that I have seen is VR accompanying MSG in MSG's solo concerts. VR even anticipates *every* one of MSG's Hindustani-inspired style of rendering carnatic krithis. Most of the other mridangam vidwans that I have observed accompanying MSG kinda grope around a few times and then catch up.
Growing up, I was so inspired by VR that I just had to learn to play the mridangam. Living in Bangalore, I ended up studying under TAS Mani for about 3 years. A far cry from VR's style of playing; nevertheless, it let me appreciate and understand different styles of playing and appreciate the kannakkus and brilliance of thanis played by PMI, PSP, PRR, UKS, TU etc.
The only downside to my total devotion to VR's style of carressing the mridangam is that I find it very hard to appreciate/tolerate the style of mridangam artists like Patri Satish (20 years back, it was Bangalore Praveen that I detested). I don't doubt their knowledge, talent and skill - it is just that they are way too loud and intrusive. I recently attended a concert of GJRKrishnan and VijiKrishnan with Patri Satish on the mridangam. He was so loud that I could barely hear the violin. Also, he kept whacking (sorry, but, that is exactly what he seemed to be doing) the mridangam so hard that it kept moving around! It sounded louder than a thavil. I wondered if an anger management class would help him.
While I have never ever walked up to musicians after a concert, I literally ran into VR once right before a concert in Bangalore and there was nobody else around. There was so much I could say to him, but, I just froze and got tongue-tied. Couldn't utter a meaningful sentence - other than a lame and embarrassing comment like "You are the best mridangam player".
One last bit regarding thaniavarthanams. I always sit through thanis in concerts and while they are always interesting, I've never heard one where once the mridangam and ghatam/khanjira artists start playing together, there is perfect synchronization - there is always dissonance between what the mridangam artist plays and what the ghatam/khanjira artist plays. Well, never - except for one concert of either Lalgudi Jayaraman or Ramani in Bangalore that had VR and HPRamachar on the khanjira. The thani was simply mesmerizing. Once VR and HPR started playing the thani together, there was not one khanjira stroke that was out of sync with VR's. It was an amazingly perfect thani.
I apologize for rambling. If I've offended any of you by stating that I find VR's accompanying superior to other vidwans, I apologize.
Extrapolating what annamalai said, MDR + LGJ/TNK + VR = euphoria.