I could not agree more.
Story of my RPing, really. First RP I ever participated in (different site entirely), it was fun at first, but no one was interacting. Eventually, I got fed up with doing what was essentially just writing a story alongside a whole bunch of others on the same thread. It just felt like an empty existence, like I was working without pay (sure, I got complemented for my writing skills, but I didn't really want or care for that). Honestly, that experience was defining, cause I recognise many RPs where it ended up the same way. I was recently in one where not one character, in the short lifespan it had, ever interacted with another that wasn't either an NPC of the player's creation or a friend they 'started off' with in the RP. It felt just as hollow as my first one, and no amount of plot or setting could keep my interest in something like that (which is funny to say, cause neither of those RPs had a true central plot that everyone was supposed to follow. It just eventuated that no one took a step towards another).
Coming off the failures of my RPing experience, I'll move onto my first truly successful one. Anyone who read the site news recently might've noticed one "Naruto: Rise of the New Generation" featured there. It's funny, I've been in more than just a few RPs, maybe not as many as some, but enough to think I'd have a varied experience, but not one went as far or as well as RotNG. In terms of plot, I daresay it was sub-par (no offense intended, good friends, but it really isn't all that stellar), it certainly didn't capture my attention in any way. However, what I did love was the interactions. And I made those myself (others were of course involved, but I was a part of those moment's genesis rather than being a spectator). For instance, during one plot arc, the village was under attack by the Hachibi, or Eight Tailed Beast (Naruto canon, if you don't know it, look it up). Boring really, none of my characters could actually do anything to it, so all I could do in direct relation to the plot was to spectate. But it was during that arc that one of the funniest interactions was born. Doing our best to ignore the spectacularly uninvolving plot (the GM did his best to involve us, but I daresay that was out of sadism rather than an attempt to make us feel special (trying to crush us with a huge beast we couldn't even hurt, how is that not sadism?)), our team was running about far from the battlefield. In one of the GM's attempts to 'involve' us, an attack from the beast found it's way coming at us. Our teacher, who had been up to this time something of a complete bastard, actually ended up taking the blow for us and saving us from harm. Of course, my character is the type to trust absolutely no one, and can't fathom this development, especially from a man known to have tried to take her life on more than one occasion. Henceforth, rather than being grateful, she seeks revenge for him shaking her view of the world. She takes him to the hospital.....while dragging him through broken glass and rubble. Later, she sends him a recording of the most annoying member of the squad talking, which was enough to make him consider suicide last time he heard it. Soon after, she sings the loudest song possible to give him an awful headache, and also to dislodge the earplugs he was using to block out the sound of the annoying member's voice. And for every action she took to punish her sensei, sensei fought back, trying to get revenge for all her antics, everyday regretting he saved them. And that was the most fun I've ever had with an RP. I didn't care about the attacking Bijuu, or the threat of war with Sunagakure, I was more interested in messing around with my team. And it was for those interactions that I put my heart into that RP.
And that wasn't the extent of that which captured my interest, oh no. Another team I was in (I have a lot of characters), leading for that matter, the leader was a conceited bitch that believed she was perfect. Two of her students were monumentally retarded, and so hilarity was born. 'Endurance training' was a common task for the students of Team Mei, involving them pulling around carts to transport their sensei, being dragged along the ground, waterboarded while someone tries to smash their head in with a mallet every time they resurface, you name it, she did it to them. Plot? Who cares, I was having fun being a bitch while my students were so happy they got to participate in 'special training'. In fact, Team Mei was one of the most active and interactive teams in the RP. Made me proud, before one of our members left for reasons.
Then there was Team Kanajiro. Now in that team, it was less funny or sadistic, but we had fun nonetheless. Our character's gelled, they got along and made friends. No, we didn't have some fancy involvement in the plot or some huge dramatic story arc, we just had our characters talk and train together. It's funny I can't find the words to say anymore, there might not have been the same memorable moments as in the other two, but it was still a fun time we had. That one had a tragic end, as the team fell into inactivity, and there wasn't anything more that could really be done. There's a new Team Kanajiro now, one I'm not apart of, somewhat sad, but I felt that it had all slowed down anyways, so it wouldn't have helped to stay on.
And the biggest mention I have to give is to my (technical) sole teammate on Team Nobushige, the one I mentioned with the delightful master-student relationship before. This teammate was of course, the annoying girl that the sensei was desperate to avoid. Now that annoying little girl, she became something of a meme and a legend in the RP. At first, all she was was a talkative girl with a lot of energy, no one would think anything much of that now, would they? But as I over exaggerated my character's annoyance at her talkativeness, the joke grew and grew to the point that everyone feared the dreaded "Motormouth Jutsu", the technique that caused a hardened, merciless assassin to contemplate suicide and take therapy sessions to cope with the stress, and brought the dreaded Eight Tailed Beast to it's knees (sealed inside the little girl, not only for reasons of power balance, but also for the sheer hilarity of seeing a small child talk a fearsome beast's ears off). And that joke, my friends, was not born of some plot, it was the product of a bunch of silly interactions between some characters.
Gosh, I rambled quite a bit here. But perhaps that illustrates the point; You can clearly see I have a great deal of love for the interactions I got involved in in that roleplay, and how not one of these memorable stories really factored into the main story (the tailed beast in the girl was more or less and afterthought, and was produced from the hilarity of our interactions not because of a plot point). Long story short, I wholeheartedly agree with the point made in the OP, and I hope my ramblings might also help convince people that a truly successful roleplay is born on the back of meaningful interactions and not a plot or setting of any kind.