Though Barbara is Batgirl here, she later becomes Oracle when the Joker shoots and paralyzes her and she's confined to a wheelchair. The controversy here is that it's also implied that the Joker sexually assaults her, so a lot of fans consider it uncomfortable or triggering in the context.
Since then, DC has pulled the cover and the artist has very graciously apologized. And I 100% respect and appreciate their decision to do so.
Personally, I can't really see that it "crosses a line", but I guess, having never been a victim to this sort of thing, I don't really get to choose where the line is drawn. Still, I think that it's a pretty reasonable depiction of something you might see between any hero-villain dynamic and the artist, Rafael Albuquerque does a bangin' job on this one.
The only thing I don't get is that now, people are slandering The Killing Joke. A lot of tweets read along the lines of "it's 2015, we can stop pretending The Killing Joke was good." and "Most people who call The Killing Joke a classic weren't even old enough to read when it came out."
If you ask me, that's kind of a terrible defense.
No one on this earth was alive when Shakespeare wrote Romeo and Juliet or when Bram Stoker wrote Dracula. Since when do you have to be alive during its time to call anything a classic?
The Killing Joke is canonically why the Joker-- the most famous comic villain of all time-- is the way he is. It's arguably Batman's darkest time; I definitely think Batman broke his golden rule at the end of the comic. It's heart-wrenching and uncomfortable and exciting.
People don't like it because, what, it depicts sexual assault? Isn't that a good thing? Isn't that giving a voice to victims? Isn't that giving an accurate depiction, an insight into what a terrible thing that is? And I think that's great, because it makes you feel things!
Say what you want about Batgirl #41's alternate cover (personally, I thought it was brilliantly executed), but do not insult The Killing Joke.