An S-corp is restricted in the total number of shareholders, and the classes of stock. It also can't have retained earnings. That's it.
S-corps aren't really popular any longer. Everyone who might have benefited from an S-corp is choosing to form LLCs instead. Less paperwork. Less formality. More flexibility. It's essentially a partnership with the limited liability of a corporation.
Heck, even AOL was an LLC for a while.