one-trick pony

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One noted for or limited to a single ability or skill.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A performing animal (especially a pony) that knows only one trick.
• noun idiomatic, by extension A person or group noteworthy for only a single achievement, skill, or characteristic.