Gradients and juicy-fruit graphics are like candy -- they provide an instant and strong positive feeling, but they don't age well. If you had to eat nothing but candy, you'd hate the sugar-coated morass your mouth had become. In general, a flat icon tends to age better and reduce "eye exhaustion" (not a physiological thing, but a psychological one). Note that this applies mostly just to icons; the entire UI is not so tightly bound by these rules.
However, design is also trend-oriented, and flat is a rising trend.
However, design is also trend-oriented, and flat is a rising trend.