These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Crowning Music of Awesome: "Ten Million Slaves" by Otis Taylor, used in the trailer and two different scenes in the movie itself (Purvis chasing down Pretty Boy Floyd and during Dillinger's second bank robbery).
Ensemble Darkhorse: Billy Crudup's performance as J. Edgar Hoover was praised even by people who didn't like the movie.
Genius Bonus / Historical In-Joke: when being taken into custody, Makley and Pierpont say they are being "shanghaiied to Ohio". This makes sense if you know the real reason why they were sent to Ohio to stand trial, which is this: Makley, Pierpont, and Russell Clark walked into the Allen County Jail in Lima, Ohio on October 12, 1933 to break Dillinger out of jail. In the process, Pierpont shot and killed the sheriff, Jess Sarber. Pierpont, Makley and Clark were put on trial and convicted in March 1934.
Karpis tells Dillinger about how he and the Barker brothers Fred and Dock are planning on kidnapping a St. Paul banker named Edward Bremer, with Dillinger turning down the offer. Though Karpis may never have actually met Dillinger, Karpis did go through with the Bremer kidnapping on January 17, 1934 - two days after Dillinger and Hamilton robbed a bank in East Chicago, Indiana.
There's also a mention during the same scene where the Dillinger gang members mention Karpis's previous major crime - the kidnapping of William Hamm in June 1933 around the time of the Kansas City Massacre.
Hollywood Homely: Billie's alleged "$3 dress" (around $50, adjusted for inflation) doesn't exactly look cheap by today's standards.
Jerkass Woobie: John Dillinger, definitely during his humane moments. Witnessing Billie Frechette taken away in handcuffs in particular, and crying as he drives away unnoticed.
Dillinger was so distraught by Billie's arrest that he considered intending to rescue her while she was being transported to St. Paul to stand trial, which everyone else in the gang - John Hamilton, Baby Face Nelson, Homer Van Meter and Tommy Carroll all were against. Van Meter, however, did know where they could get vests. Indeed, that Friday morning, a few days after Billie was arrested, Dillinger and Van Meter went to Warsaw, Indiana, where they took a police officer, Judd Pittenger, hostage (there was a brief struggle where Pittenger tried grabbing the barrel on Dillinger's Thompson, and Pittenger was then pistol-whipped by Van Meter) and walked him to the police station armory at gunpoint, where they took a number of pistols and bulletproof vests.
Tear Jerker: The ending is surprisingly good at making you forget that Dillinger was in fact a ruthless gangster that regularly killed innocent people. However, Dillinger's death scene is also surprisingly accurate.
Except he didn't. In truth Dillinger was a blue-collar crook who planned his robberies around not killing people. The only time he ever killed someone was on January 15, 1934 when he lost his temper during a bank robbery in East Chicago, Indiana and gunned down a police officer named William O'Malley. Since he never was convicted, whether Dillinger was the robber who gunned down O'Malley is sometimes questioned.
He was in it for the money, the thrill, and as revenge against a corrupt system he felt betrayed the common man like himself. Dillinger despised cold-blooded killers like Bonnie and Clyde and Baby Face Nelson because he was wise enough to know that he was a Villain Protagonist and the people coming after him were just doing their jobs.
Dillinger hated that he had been forced to work with a murderous bastard like Nelson later in his career, and never bothered to try and hide it. He even threatened to kill Nelson himself if he shot at anyone needlessly. This once happened when Nelson wounded a deaf man during their robbery of the First National Bank in Mason City, Iowa on March 13, 1934 - ten days after Dillinger escaped from Crown Point. Dillinger called Nelson out on this. This resentment was mutual: Nelson hated the fact that Dillinger got all the attention, and how the press were drooling all over him (Dillinger's exploits were popping up in every major newspaper across the country; the papers were so obsessed with Dillinger that every reported sighting was grounds for a new front page article, quite a contrast to Bonnie and Clyde.
Actually, a few Dillinger robberies did see officers get shot. Not by Dillinger, though. The shooters were usually either Nelson or Van Meter.
But the film version was a ruthless gangster who killed innocent people.