Well. Everything and nothing.

Public intoxication was always a good one, chronic absenteeism at your job was another potential minor infraction that could get you sent east. Having a relative living overseas, receiving mail from someone outside of the Soviet Union, being openly religious, telling the wrong joke or making the wrong comment to the wrong person, all could easily get you denounced and shipped out. Of course, you could actually not do anything, simply being denounced by a Party member could have you arrested and sent into the labor camps. So you could be sent to the camps for something as simple as being tipsy in public to being 100% innocent. There is the apocryphal quote from Stalin where it is claimed he said "If we only arrest the guilty, then the innocent will have nothing to fear."

Side note: The majority of the Gulag population was not intellectual/political dissenters, but by in large ordinary criminals. Of course, (if) once released an ordinary criminal is less likely to write a memoir about it, so there is a skewed view that the camps contained large numbers of political dissidents.

(My MA thesis was on the female population of the Gulag during the Stalinist era.)