First off, I like the Walz pick a lot. I think it helps shore up the base with progressive voters and gets them reengaged into this election. There will always be a segment of the left that will find qualms with any Democrat not named Bernie, AOC, Rashida, etc. Given Tim Walz list of people-focused, progressive accomplishments as Governor of Minnesota (free lunch program, gun safety reforms, public education spending, child tax credits, etc.) I think it's enough to excite the progressive/left base of the party to get out and vote for Kamala.
The other big thing I like about Walz is his appeal to men, specifically white working class men. While we will never win a majority or anything close to even with white working class men, Walz does give us a chance at holding onto to some of the ones Biden won back. He can the Hank and Peggy Hills of the world that might not vote for Trump but aren't exactly excited to hold their nose and vote Democrat. He also overperforms, to an extent, with working class voters in rural America. I still don't think many of those voters will vote for a black women named Kamala, but picking Walz certainly doesn't hurt.
Walz has that authentic, folksy, everyman kind of persona. He has a record of service in the military, he was a HS football coach, he's a dad. As a friend said to me this morning, he seems like the kind of guy that actually likes going to the state fair and eating a corn dog, riding the rides, and being there. Not a guy whose only doing it for the photo opp. He also neutralizes JD Vance's military service record because he has served himself.
I don't know if there is one, singular, big knock on Tim Walz. Certainly nothing that compares to what Trump has done. I think their knock on Walz will be more of the same they've already been attacking Kamala on. Too far to the left, too radical, a marxist, bad on the border, responsible for high inflation, etc.