As announced by rules judge Shelden Mennery in the February 2014 Commander Rules Update, Sylvan Primordial is Banned in Commander/EDH.
From the official rules release:
Sylvan Primordial
We feel Sylvan Primordial is causing far more problems than its contributions justify, and that the format will be better off without it. It meets many of the heuristic markers for a banned card, insofar as it invalidates many other creatures as search targets and causes arguments about whether its use is degenerate or reasonable. It can be easily accelerated into on turn 4 or 5 (before players are expected to have extensive defenses or threats online), at which point it turns a reasonable ramp deck into uninteresting games.
If the card was just a big ramp, or just utility destruction, or just spot land destruction, it would likely be fine but by combining both factors it becomes ubiquitous, frequently overwhelming, and repetitive. After some debate in previous seasons, the committee members all voted in favour of removing it from the format.