Theodore Roosevelt is well-remembered for many of his accomplishments, but he is also the subject of much criticism. In addition to his involvement in the Panamanian Revolution and the canal, thoroughly described in the other answers here, let's look at some of the other common criticism of this president.
Some critics of Theodore Roosevelt point to his excessive use of the executive order. While president, he issued over one thousand executive orders. By contrast, President McKinley issued only around one hundred. Many of Roosevelt's orders were over trivial matters. He also used this power to create vast swaths of federally protected conservation land. Regardless of its effects or motives, many point to Roosevelt's many executive orders as greatly expanding the president's power well beyond what the Constitution originally intended. He frequently inserted himself in the debates going on in Congress. This was something that many felt was contrary to the notion of separation of powers on which the federal government is structured.
Roosevelt is also criticized for his aggressive foreign policy. He was president at a time when many in the United States felt that the country should exercise its power overseas. Using the Monroe Doctrine as a blueprint, Roosevelt issued a corollary to assert the country's prerogative to intervene throughout the hemisphere. While many supported this move, there was also a strong isolationist movement in the country that saw it as overly aggressive and against the country's interests.
On a personal level, Roosevelt has been criticized for his racist views. He believed that the white race was the superior race to all others. He used this view to justify the country's imperialistic actions abroad and to ignore the plight of minorities at home.