Shields is the last of your standard, reasonably athletic, hard worker who makes up for their lack of elite ability with grinding pressure. The last of these guys have recently checked out (Forrest & Bonnar), with a stragler or two who are still around but have yet to scan their exit passes (Sanchez & Fitch).
He just doesn't have, or has never had, that obvious athletic factor to really leave waves in the sport. His BJJ is top notch, but he's no Andre Galvao. His wrestling always seemed elite, but that's because he was usually out-wrestling guys like Daley or Lawler. He had great match-ups in his career. Hendo is probably his most prestigious win, but I'd almost write that off as a bad cut for Hendo as he has never been outworked like that until he stepped in with a much larger and decorated Cormier.
Still, Shields has achieved enough to warrant his spot as one of the top WWs and (not so much) MWs.