the lack of deal enforcement
the lack of verifiability of N.Korea's denuclearization efforts as part of the "Complete, Verifiable, Irreversible Denuclearization" or CVID protocol
the legitimization of North Korea's regime
China's role in the process
the end of joint military drills with South Korea.
While Trump's response to most critics was that the process is just starting and that it will take time to denuclearize, where Trump will see the greatest amount of pushback is on the last bullet point. Speaking in an interview with George Stephanopoulos shortly after the one-on-one with Kim, when asked if there was talk of pulling U.S. troops out of South Korea, Trump said the topic didn't come up, however he said the following:
"We didn't discuss that, no. We're not going to play the war games... I thought they were very provocative. I also they're also very expensive."
In his press conference after the summit, Trump reiterated that the US would halt joint war games with South Korea, as Pyongyang has agreed to destroy a "major" missile testing site.
"We will be stopping the war games," Trump said Tuesday during a news conference after four hours of meetings in Singapore with the North Korean leader. He offered no specifics about which exercises would be affected,.
According to Bloomberg, Trump's announcement came as a surprise to the South Korean government.