Just goes to show that beauty is in the eye of the beholder; there are a few things about it I do like. I like the really small avatar (their users don't, but then if you're used to a massive avatar it would be a shock!) since this reduces postbit space even more to focus on content not stats. I like the simplified flags under each post (off topic, agree, disagree, like) plus the mini-avatars of each post rater. I like the general style, light and easy on the eye.
I don't know anything about the platform, but reading the user comments it looks to have been a big step backwards in terms of features. I think that is their mistake rather than the style - you take away features from your community at your peril.
Like many here I can't understand their reasons for moving. It is simply wrong to suggest that a large forum is unstable on Xenforo since there are so many that run fine. Surely they just needed to sort out better hosting and with Mike Edge amongst others seemingly offering blazingly fast hosting for peanuts why change to Vanilla? Seriously
looking at their pricing, to get daily backups and just 300k page views a month (which is nothing in forum terms, as each single visitor can create hundreds a day!) it's $149 a month - you could get some amazing hosting for that with money left over. Sadly (for Xenforo) I think this is yet another casualty of people getting a bit concerned with the legal case and lack of updates, it seems to be the straw that is breaking the camel's back in some cases.