Law & Order is the kind of show that's formulaic for a good reason. If it didn't stay pretty close to its standard pattern, and if it abandoned all of its cliches, you can bet that fans would scramble to find a better program to watch. Some of these important elements include the distinctive theme song, the breakdown between a police portion and a courtroom portion, cop partnerships, lawyer partnerships, backroom dealing and the twist halfway through when you realize the guy under arrested isn't who committed the crime.
Of course, there's always that sound ("dun dun!") as well.
If you're a fan of Law & Order or any of its various spin-offs, you better check out David Rees' song below (via Vulture). The creator of the webcomic Get Your War On (as well as the proprietor of an artisanal pencil-sharpening business) has outdone himself. The song, besides being catchy and well-written in its own right, covers all the bases when it comes to Law & Order fandom.