Brandon Belt is the type of player that fans of every team hope to see emerge from their systems. Someone who is initially undervalued, but adjusts and becomes successful in a way that should be sustainable because it is fundamentally sound. He is young, and his best years are still ahead of him. That should be enough to make people uniformly excited. The problem is expectations. San Francisco has a lot of fans with really unrealistic expectations. Here is my attempt to explain.
At least regarding prospects, in the post-Bonds rebuilding period between 2005 and 2010, everything was coming up roses for a little while...
Cain → superstar
Lincecum → superstar
Sandoval → superstar
Posey → superstar
Bumgarner → superstar
Belt came up at the tail end of that. Some of these guys may have passed under the radar as superstars in the national sense, but they were killing it on the field and we held them in ridiculously high esteem. The big difference that is lost on fans is that all of those guys started out as mega-prospects, whereas Belt was drafted in the 5th round. You could conceivably compare him to Sandoval, who also "came out of nowhere", but Sandoval had an approach that defied development. Belt had an approach that REQUIRED development. Appropriately enough considering his hitting style, patience was always going to be necessary with Belt.
Also, consider that Belt came to prominence in prospect awareness during the 2010 season. With the success they were achieving, the Giants really built their fan base suddenly and massively. You have a bunch of new fans who are accustomed to every rookie becoming AWESOMENESS INCARNATE. Dominant pitchers! Insane hitters! And then there's Belt, whose best tools are arguably his pitch recognition and his defense. He has medium power that could still grow, and he's got decent speed on the bases. He is an above average player, but he lacks the flash of those other guys. So people are like, "What the hell? We've come to expect GREATNESS!"
Once that mindset is achieved, it doesn't take much to make the next leaps: every strikeout looking is proof that he isn't an aggressive enough hitter. Every sad expression is proof that he doesn't have the guts to play ball. And, importantly, every failure threatens the notion that the Giants are a magical creature that cannot fail. Remember: many of these fans have not seen failure. They don't know about the long line of AAAA players that couldn't hold a candle to Belt. To you and me, Belt is the player that we hoped for when we saw Niekro, Ortmeier, Bowker, Lewis, Ishikawa, etc. To them, these people never existed.