Many things... For one, dismissing Calvin as a revolutionary who ran a police state in Geneva. The town council was the civil magistrate. He was the town pastor. The relationship was often uneasy and at one point he wanted to resign but was refused.
There are a lot of Catholics who read here, and the point isn't to start theological discussion, but if you actually read Calvin you'll see that his focus was on Bible exposition and systematic theology (more Bible exposition). I get that Catholics aren't big fans of Calvinism, but it's not a fair analysis to cast Calvin as a Judaizing revolutionary running a police state. It's dismissing a formidable intellect, much like I would be if I called Jones a "reactionary crank." Similarly, Jones's views on Luther are not balanced, although, to be fair, Luther was a hothead.
That the Puritans were Judaizers... They saw themselves as eliminating the Judaizing of Rome on the Gospel. It's long been a question of what obligation we have to follow the OT law. The landing place for classic Protestants is that the ceremonial and judicial laws are gone, but the moral law as expressed in the Ten Commandments, remains (including for many the Sabbath) and should guide us into good works. Calvin called this the "Third use of the law." That's a short and inadequate summary.
It's a bit off-topic here, but Calvin might have been a "Judaizing revolutionary" in the most literal sense, as a French converso, his name, officially registered as "Cauuin", and was known as "Cauin", which is the French transcription of "Cohen". I haven't researched that enough yet but other serious researchers like Eustace Mullins did:
Eustace Mullins in The Curse of Canaan (1987), Chapter 4, p. 84, (Library of Congress Catalog Card Number 87-90479) provides the following pertinent information on Calvin:
"... He was educated at the College du Montagu, where Loyola, founder of the Jesuit [Roman Catholic] sect, had studied. Cauin later moved to Paris, where he continued his studies with the Humanistsfrom 1531-32. During his stay in Paris he was known as Cauin. He then moved to Geneva where he formulated his philosophy known as Calvinism. At first known in Geneva as Cauin (the usual pronunciation of Cohen), he Anglicized his name to John Calvin."