Aside from sexism and racism, which is kind of expected in old books, a big problem is they tend to feature (and either actively glorify, or gloss over/act indifferent) predators. Adults dating or marrying kids or teenagers, or adults who are attracted to teens, and they're treated as heroes to root for or neutral characters, where if anyone criticizes the relationship, it's for something other than the fact that the adult should be behind bars and on a list.
There are a lot of old books that have good stories, but are outdated as hell. For example, I can't read Anne of Green Gables because it features some predatory relationships (Rilla gets with a 22 year old, and fully grown teacher Mr. Philips is attracted to Prissy, a 16 year old kid), Rebecca of Sunnybrook actively again glorifies and tries to have you root for Adam, a 30 year old who actually gets jealous when he sees a high school age Rebecca just talking to a boy her age, even Little House on the Prairie and many Austen novels glorify this.
Lolita is the only book I can think of that actively calls out adults who are into minors.
by AlboGreece