1) The Cast
The cast for Fantastic Four got a lot of heat from fans for not being exactly like their comic counterparts. Most notably attacked was Michael B. Jordan as Johnny Storm/Human Torch, due to his skin color, even though Idris Elba (Heimdall in Thor) and Vondie Curtis-Hall (Ben Urich in Daredevil) were both race-changed in the Marvel Cinematic Universe to no complaint.
But the fact of the matter is this was actually a great choice of actors for the film. As stated before, Trank and Kinberg were basing this iteration of the property off of the Ultimate Fantastic Four line of comics, which depicted all four of them as young adults, as opposed to the older adults in the traditional comics. This meant that Reed Richards did not have the grey side burns, Sue Storm had no children, Ben Grimm was not an Air Force pilot, and Johnny Storm remained in high school.
To add to this, these were not newbie actors taken fresh out of drama school. All four were well established, backed by a host of strong performances in films and television shows. In short, everyone brought their A-game to Fantastic Four, especially with the amount of pressure put on them.
Not only that, but each actor fit the personality of their respective character so well: Miles Teller was withdrawn yet intelligent like Mr. Fantastic, Kate Mara was smart yet cautious like Invisible Woman, Jamie Bell tough yet full of heart like The Thing, and Michael B. Jordan hot-headed and wisecracking like the Human Torch.