Not everyone who enters the WWE can be a superstar. And even fewer will ever get an opportunity to be one of the "top guys." There just aren't enough spots at the top of the pyramid for every talented performer to get their due. Still, it's unfortunate just how many premier talents have come and gone in the WWE without ever getting a significant push.
The Ruthless Aggression era was particularly cruel to un-established talent. Following the buyout of WCW, the WWE basically doubled its roster, creating a dearth of available screen time for everybody. The much maligned Invasion angle also brought in a number of ECW superstars, making it next to impossible for writers and bookers to give every budding performer the attention they deserved.
As the talent pool expanded, characters and storylines started bleeding together while quality performers were getting swept under the rug left and right.
Looking back, it makes sense that the Ruthless Aggression era is one of the most under-appreciated periods in history, since that describes about a quarter of the roster at the time, as well.
That's not to say that everyone on this list deserved to be the WWE Champion. But they most assuredly deserved a much better place in the company than what they got.