The standout for me is the focus on "less is more". Keaton never spoke a single syllable more than he needed to. He was succinct, but also had an intensity behind his eyes to supplement his silent nature. It's most definitely not a "loaded" performance, but neither is the character. We're talking about a man whose dialog is most commonly represented in the books through inner monologues. It's just not going to translate to a performance which is outspoken. Nor should it, imo.
By itself, Keaton's performance isn't some outstanding marvel. Truthfully I've only come to appreciate his acting the more I see other actors try on the cowl. They really all pale in comparison, and to me it speaks volumes of how hard the role actually is beyond the surface. The likes of Kilmer, Clooney, and Bale have all turned in several great performances over their careers. But they've all been unanimously poor Batmen in my eyes. So there's clearly something that isn't being easily harnessed.
With Ben purportedly being a reader (likely in the 70s and 80s) for a while, I'm hoping he's taking a few pages from that era. That's when Bats was arguably written at his best.