"There has been diminishing hope and optimism about American policy in the Middle East in the past month," he told participants at a workshop of the forum in the Qatari capital.
On June 4, Obama called for a fresh start in relations between the United States and the Muslim world, in an apparent break from the policies of his predecessor George W. Bush.
Washington led the invasion of Taliban-ruled Afghanistan in 2001 and of Saddam Hussein's Iraq, another Muslim country, two years later, under Bush.
Talhami said there was a general "frustration about the Obama administration," due mainly to the lack of progress in the Israeli-Palestinian peace process.
Obama's election as US president had a "tremendous positive impact" in the Arab world, said former Egyptian diplomat Nabil Fahmy.
But positions adopted since his election have been "disappointing," he said, pointing to Washington "backing down on (Israeli) settlements (in the West Bank) and saying it is up to the Palestinians to take steps," he said.
The US president on Saturday reiterated his commitment to work towards ending the Israeli-Palestinian conflict based on a two-state solution, in a video address to the forum.
He also named a special representative at the largest pan-Islamic body -- the Organisation of the Islamic Conference.
"It has only been eight months since Cairo, and much remains to be done. But I believe we've laid the groundwork to turn those pledges into action," Obama said.
Prominent Egyptian-US human right activist Saadeddine Ibrahim said Obama had so far been "a big disappointment" in the Arab world.
"People expected action to follow the (Cairo) speech ... They expected Obama to exert pressure on the Israelis but he did not," he said.
"The United States took some action, like the appointment of (Middle East peace envoy) George Mitchell, but this has not produced results," he said.
Ibrahim, however, said Obama was still "more positively perceived than other US presidents."
Mitchell completed last month a round of shuttle diplomacy between the Jewish state and the Palestinians and proposed an initiative to relaunch peace talks.
The plan calls for Israel to loosen its hold on some Palestinian-controlled areas, release a number of prisoners and ease a virtual blockade of the Gaza Strip, according to a Palestinian official.
But the "indirect talks are a regression after 20 years of proximity talks," chief Palestinian negotiator Saeb Erakat said during the round-table discussions in Doha.
Israeli Prime Minister Benjamin "Netanyahu is the one placing obstacles in the way of the Obama administration," he said. "You (Obama) need to answer us now: why the US cannot recognise two states now?"
"It is time to move in this direction. You want to defeat extremism in the region, the key is here. If not, extremism and (Al-Qaeda leader Osama) bin Laden will be winning," he said.
---------------------------------------------------------------