#2. Sting
There was so much build-up and so much anticipation for Sting's debut in the WWE and when it finally happened it was a huge, and I mean huge, deal. Finally we were going to get all of the dream Sting matches no-one believed would happen inside of a WWE ring.
What happened next was incredibly underwhelming, but through no-one's particular fault as Sting only had two PPV matches before the same cervical spinal stenosis that had put Edge out of commission had also forced him to retire as well.
The first was against Triple H at WrestleMania in a losing effort and the second was for the WWE World Heavyweight Title against Seth Rollins, again in a losing effort. During the Rollins match, Sting suffered a neck injury, but was able to finish the fight. Shortly afterwards he was forced to retire.
This was almost certainly too soon because it meant that we didn't get to see Sting win a high-profile match or feud and it also meant that we didn't get the once-in-a-lifetime dream match of Sting Vs. Undertaker. A match so wanted that even now people cling on to hope that it'll happen.