As someone who works in web design i'm having second thoughts even now about typing this message because I know the work involved and it feels like a slap in the face to the staff here, but personally, i'm pretty dissapointed. All of the charm and character the site had is gone, and it now looks like every other stock XenForo board on the planet. From a design standpoint it's like unflavored oatmeal, plain, consistent, and entirely unremarkable.
From a technical standpoint, yeah, it's great. I'm sure it'll be easy to use on mobile and it seems pretty fast. I never had issues with the old site on desktop so I can't really compare, but it's true that XenForo scales better across multiple devices, i'm not ignorant of the many improvements. The content is still the same, the people are still the same, and ultimately that's what should matter the most, but the old forum was a joy for me to open up and browse through and now the stark white drabness has me doing a double-take to remember what forum i'm on because I belong to six others that all look completely identical.
I'm not much of a contributer, I know that, I mostly come here to read about what other people are working on so I doubt what I have to say will mean much. In the 10 years i've been here the site has changed several times so this isn't my first rodeo, and it's not that i'm unwilling to get used to the new layout, I just think that it looks... bad.
Nothing at all wrong with holding this POV (and I greatly appreciate you sharing it in an even-handed way). You are largely right. This design IS very vanilla, which on the face of it, might feel counter-intiutive, considering this is a site largely for artists and creatives, but I have always looked at the RPF as a container for greatness, not greatness itself. It is simply a vehicle for creative people to share their amazing work... and i could keep going down that road for a while, but the other side of that coin is the one you have pointed out very thoroughly; we want the site to be exceedingly nimble, easily upgradable (there is already an even newer version of XF that is even faster and MORE mobile friendly) and we don't want to wait another 10 years before moving to it. This was a VERY VERY complicated and difficult transition and the members have had to suffer through YEARS of a barely functioning site because we customized the last one too much. We don't want to make that mistake again. Those are the real reasons behind the stripped-down approach. With that being said, I am always open to suggestion.