I apologize, again, for the lateness of this post; I was at the beach, and thought I would be able to get online wirelessly, but the connection was too bad.
One of the most obvious aspects of polytheism is that we generally accept that other people’s deities are as likely to exist as our own. The ancient Greeks, as is well known, syncretized fairly relentlessly – many of the epithets of the Gods (e.g., Zeus Ammon), reflect the reconciling of “foreign” gods with or as aspects of the Theoi, and in some places during the Hellenistic period this syncretism approached full-fledged synthesis (I am thinking here primarily of Alexandrian Egypt and, to a lesser degree, Judea under the Ptolemies).
The Japanese also have a long history of syncretism – shortly after Buddhism was introduced in the 6th century, the Japanese began to syncretize Buddhist saints as analogous to Shinto kami, a process that culminated a hundred years later, during the Nara period, in the development of the concept of shinbutsu shugo – the full-blown synthesis of Shinto and Buddhism. This resulted in the addition of Buddhist statues to Shinto jinja, and the installation of shrines to various kami in Buddhist temples. 
(There was an interesting article on this recently in the Japan Times.)
Despite a temporary setback during the period leading up to WWII, when the government suppressed Buddhism in the name of State Shinto, this synthesis persists to the present – most Japanese still consider themselves to be both Shinto and Buddhist (or, probably more accurately, to perform both Shinto and Buddhist practices). 
I sometimes wonder what Western civilization might look like today if Christianity had successfully synthesized with the native polytheistic religions of Europe the way that Buddhism did in Japan; sadly, we can only speculate. However, I believe we *can* look to the Japanese experience for insight into possible ways to move forward into the future.
 http://www.religioustolerance.org/shinto.htm; see the section titled “Number of Adherents”.