If characters were the only thing the universe had to offer, that would be bad, but I don't think it's true. There's the quest for finding humanity a place in the galaxy as a newcomer. There are the alien races (races aren`t characters) and their politics. There are struggles and conflicts like the Genophage, the Quarian/Geth conflict, human expansion and Batarian resistance to it. There is space exploration, visiting known star cluster, exploring pristine planets as well as developed colonies and learn more about these places. There are mysteries to unravel, the thorian creature, the Leviathan, the Prothean empire and of course the Reapers. There is so much more the universe has (or had) to offer and this is what made the games for me.
I was responding to that person saying they lost interest simply because there won't be Shepard or the familiar characters. So I was using a hypothetical where everything else was the same.
True, the races have some interesting history, but ultimately they would be boring without the characters we interacted with. The Elcor wouldn't be interesting without the Elcor ambassador or the jerk with a cigar on Omega. The Quarians wouldn't be interesting without Tali or Xen. The Krogan wouldn't be interesting without Wrex, Uvenk, or that bomb maker. The Asari wouldn't be interesting without Liara and Aria. The Turians wouldn't be that interesting without Garrus and the Turian Counselor. The Salarians wouldn't bet interesting without Kirahee and Mordin. Would you care about the Genophage if not for Wrex? Would you care about the Quarians if not for Tali? Would you be interested in anything but wiping out the Geth if not for Legion?
What is Feros without the Thorian, Jong, or Shiala? What is Noveria without Anoleis, Parassini, or the Rachnii Queen? Those places wouldn't be interesting as mere Codex entries.
The characters keep the races from just being groups of people that did something at some point in history. They make that history matter.
I did like the exploring planets, but I wish there was more to find. Little landmarks that told me something about the planet, not a crashed satellite that had random pieces of gear. I thought they were too big for how little there was on them. That said, I agree that flying around exploring the galaxy was cool in theory. We barely touched on the Prothean Empire, which was a huge mistake given their importance to Humanity and galactic society. That should have been a bigger part of ME2. After all, the Prothean beacon visions and the Cypher were what made Shepard unique. That was scrapped in ME2 in favor of "he's an icon" which was never realized in the plot.
While I actually did like the investigative nature of the DLC, Leviathan was a late and weak explanation for Reaper origins.