It is hard to list them all but let's start with some rabbinical opinions that have been largely ignored or abandoned.
The Ramban opines that ONLY red wine is Kosher for Qiddush, and that white wine is Passul. (Pink? Don't Ask ;-). While this position is the preferred position for Arba Kossot at the Seder, it is by no means normative year 'round.
The debate regarding rennet less hard cheeses continues. A minority of Rishonim - viz. Chachmei Narbonne - have exempted vegetarian rennet from Hazal's G'zeira against Gevinat Aku"m.
OK - individual rabbis are not infallible. After all even Aaron corrected Moshe in parshat Shemini! However, a "group" a kat, is now claiming that any post-Talmudic decision is inherently fallible. No matter how accepted. No matter how popular. No matter how much history is behind it.
Selichot are now optional - if not an egregious example of Bal Tosif. Fuhgeddabout Tashlich. Why blow Shofar in Ellul? IOW the entire Post-Talmudic evolution of Halachah and Minhag is now "optional"
We may object upon many grounds. But, being Ellul I will focus upon the character trait of Ga'ava - Hubris.
A historically approved and sanctioned practice - be it halachah or Minhag - has the tacit approval of the collective Rabbinic Minds of the ages. Of course there exceptions! Some Minhaggim that have been constantly debated - and therefore are another matter
Rather, I am referring to practices that are NOT debated, or are accepted and merely the details are quibbled.
A The completely debated:
Qitniyyot on Passover has always been controversial. Modern dissenters have a history of rabbinic support for their opposition. Modern Protesters may be rebelling, but hubris would not necessarily apply.
B Accepted but Quibbled:
Miqva for men
Rema AFAIK requires Miqva only once before Yom Kippur. One dunk. Many add Rosh Hashanah. Many dunk 3 times.
The dissenters against Miqvah are imho displaying hubris because the principle of Miqvah has been universally sanctioned. Only the "why" is contested. For Rema it is about Tum'at Qeri. For others it is about Teshuvah.
C. The Universally Accepted
Shofar during Ellul
As far as I can tell [AFAICT] Shofar in Ellul meets this criteria. Who would dispense with this as non-Talmudic when AFAICT every community has practiced this "forever"
Add to this the authority of the Shulchan Aruch. Certainly there are individual dinnim that have been ignored or protested. But there is a movement to simply discredit the entire enterprise! The essential SA is not in question - only quibbles on certain passages. Yet some fundamentalists casually toss it aside.
This also is beyond simple rebellion, this is a case of Hubris.
We can quibble with a Taz when the Shach shows us another way. And when Rema states something and the chorus of Nos'ei Keilim protest, we certainly would be prone to set aside that decision of Rema
But to toss out the enterprise? Hubris! I say, Hubris!