Taft did a good job of not making it publicly known what his political stance was on many of the political issues of the day. It must have been presumed by Roosevelt, perhaps it was even discussed, that Taft would continue his work of reform (the progress) that he had accomplished.
Roosevelt was under the impression that Taft was, like him, a progressive. In fact, Roosevelt even had him make a pledge to do just that.
What Roosevelt did not know was that Taft was a classical liberal (a conservative by today's standards) who secretly despised Roosevelt's idealist agenda. In fact, he would later claim that Roosevelt's disregard for the Constitution made him "not unlike Napoleon."
So, in essence, Taft reneged on his promise. But not initially. He initially did some "trust busting" of his own by initiating 80 anti-trust suits. However, he later backed away from these suits, and aligned himself with the more conservative members of his party, as opposed to Roosevelt's progressive wing of the party.
Despite being a conservative, he agreed to sign the Payne-Aldrich Tariff in 1909, which essentially raised some tariffs even higher than they were during the Roosevelt administration. Roosevelt actually wanted him to veto this bill.
While he was president two very progressive amendments were ratified.
- 16th amendment, which called for a progressive income tax. This allows Washington to tax income and not allocate it equally to all the states. This allowed the president and Congress the ability to pick and choose who to give the money to, which was a direct violation of the Constitution.
- 17th amendment, which called for the direct election of senators. This made is so that no longer could state House's choose the senators, instead they were voted on by the people. This made it less likely that senators would defend and protect state's rights, which was why it was set up the way it was. This made it much easier for progressives to get their agenda approved even when it directly vioilated state rights (Obamacare is a great example)
Teddy Roosevelt was irate to learn that Taft had overturned many of his reforms, and he decided, still in his early 50s, that he still had the energy to be president. Since the republicans nominated Taft, Roosevelt ran under the Bull Moose Party as an ardent progressive.
Roosevelt was even more progressive than democratic nominee Woodrow Wilson. Because the republican party was split, Wilson easily defeated both Roosevelt and Taft.