The money.
Fans expect players getting paid that much to do as well or better than they were doing before the signing, even if that is unrealistic.
I was okay with the signing, as I felt he was likely the best or one of the best FA Sp'ers on the market for a 5-7 year stretch. We needed an ace. There he was. Had we signed Scherzer the prior season (instead of Pablito and Ramito), the fans would be happier.
Many felt the deal was too long, but a necessity to land a gem like him. We expected he'd give us some great years early on and then probably be a burden towards the end of the deal.
It turns out we mitigated some of the back end losses, but paying $16M a year for nothing is still a large sunken cost.
Now, did he give us even close to what we expected for the first 4 years? I guess it depends on each of our expectations, but I think it's safe to say almost all of us thought he could have given us more. Without that playoff performance, we might be nearly unanimous on this (year, right!).
I won't dive too deeply into the numbers, but he gave us 98 starts and 588 IP in 4 seasons That's about 24 starts and 147 IP a year. While that doesn't seem awful, the guy had been a horse before the signing:
GS/IP (reverse order before our signing him)
32/220
34/248 (led league in both)
27/187
31/211
34/224 (led in GS)
32/209
(He did lead the league in IP his first year with the Sox, but the 358 IP the last 3 years combined really hurt. That's less than 120/yr.)
ERA/WHIP
3.38/1.199 first 4 years with TBR
2.90/1.087 middle 4 years with TBR & DET/TOR
3.84/1.204 with Boston
ERA+
117 first 4
133 middle 4
118 last 4
I realize it's not all about numbers, but I for one, expected better in those 4 years. He wasn't bad. It wasn't a bust, but at the price, Price was not worth the price tag.