Response from the OK Kosher Certification:
Historically, Orthodox Jews have drunk beer without any kosher certification. In our times, most people look for a kosher certification even on water!
Beer is a lot more complicated than water and there are various kosher concerns that can come up in production.
Domestic (American) unflavored beer should have a kosher certification, even those produced by the major brands.
The laws in Europe are very strict when it comes to the manufacturing of beer; however, in the United States the manufacturing process is not so heavily dictated by the government and there is a lot more room for ingredient variations.
European beers (produced in Europe) do not generally have these ingredient issues and, therefore, would not have a kashrus issue.
Domestic light beer should never be drunk without a reliable kosher certification and the same applies to domestic flavored beers or beers from domestic micro-breweries.
There is a reason that micro brews and flavored beers have such unique tastes, and it’s not only due to the way the hops are fermented. Most of these specialty beers include a variety of added flavorings that are quite kosher sensitive.
All domestic light, flavored and micro brews definitely require reliable kosher supervision.