What you're seeing Brandon Belt do in spring training (.417 average, four home runs, 1.294 OPS) is exactly what he did in the minor leagues -- he had a .343 average over two seasons, with a 1.052 OPS and 27 total steals over 670 at-bats.
The major-league version of Belt has been less impressive for the San Francisco Giants: a .259 average and .762 OPS in 598 at-bats. Belt isn't incapable of performing, or even excelling, at the major league level. He was bounced around between outfield and first base in 2011, and started to hit a stride last year, but was still bumped occasionally so Buster Posey could play first base. In other words, his development could have been stunted a bit by all the moving around and somewhat erratic playing time.
Belt is still just 24, and that power could start popping this season. He currently has an ADP of 262, which will probably start to rise as more Fantasy players take note of the spring stats and dig into his minor league past. Consider him a viable starting first base candidate in mixed leagues. And if you do that, you can use those early round would-be first base picks to strengthen other positions.