For some, it was probably the very minor on-screen time the character had in the movie. For others, it was probably the outcome (or maybe lack of?) of Leto's method-acting for the role.
But even Leto himself has some thoughts about what fans saw onscreen. Speaking to BBC Radio 5, Leto confirmed that there were much more material filmed for The Joker. In fact, it's enough to have an entire Joker movie out of it.
"When I read the script, the Joker was kind of a parallel story. My character was out to rescue Harley and get Harley back from the Suicide Squad… I brought so much to the table in every scene. It was probably more about filtering all of the insanity. I wanted to give a lot of options I think there's probably enough footage in this film for a Joker movie. If I were to die tomorrow, maybe the studio would roll something out. A Rated-R or a Rated-X performance in there somewhere."
This is surprising, considering that he would be out of the screen for the biggest chunks of the movie. Leto did point out that the movie may have told The Joker's story in another parallel instance, instead of making it the main plot.
While we do see a lot of the Joker-Harley dynamic in the movie—at least for when the Joker is actually on-screen—perhaps The Joker was not able to show the extent of the madness in the final cuts.
Maybe part of this is in line with the one regret that the actor had. That is, the lack of an R-rating.
"I always wished this film was rated-R. If a film was ever gonna be rated-R, it should be the one about the villains."
Whether or not we'll still see more of The Joker somewhere in the DC cinematic universe, or even with a potential team-up with Batman, as Leto suggested, remains to be seen. But Leto himself sad that he feels that there could be a lot more to come--without committing to any post-Suicide Squad movies.
Suicide Squad is now out in theaters.