9. He's Both An Intellectual And Physical Threat
By the time Murdock has his own law firm by day and is patrolling the streets at night, Fisk has amassed a business empire with ties to several criminal organisations, and everyone from reporters to judges on his payroll. Consider that he also speaks fluent Mandarin and Japanese, and is able to read people enough to know when they have a .22 handgun in their purse, and even Aldrich Killian would be intimidated.
In terms of the tools and alliances he has built up from such a humble background however, it is not that he has them that is the threat, but that he is clever enough to know how and when to use them. Something which is unparalleled when combined with his phenomenal physical strength.
With more emphasis on life on the streets as opposed to improbable scientific research, Daredevil may be lacking in superserum and mechanisation, but Fisk is an adversary no worse off for this. By the time Murdock finally comes to blows with him, we have already seen what damage this beast has done to those who stand in his way, with or without the aid of a car door.
The MCU has seen villains that show great intelligence (Arnim Zola) or strength (Abomination), and it's not that there aren't those with both, but none which can match the quantities of both as shown by Wilson Fisk.