"There was nothing new in the speech," Zaki Bani Rsheid, head of the Islamic Action Front's political office, told AFP, a day after the king's first televised address since pro-reform protests started in January.
"The king has expressed hopes, as we have heard several times in the past, but he did not give specifics and there were no guarantees."
King Abdullah on Sunday pledged a new electoral law that he said would result in "a parliament with active political-party representation ... that allows the formation of governments based on parliamentary majority ... in the future."
The opposition, particularly the Muslim Brotherhood and its political arm the IAF, have demanded sweeping reforms that would lead to a parliamentary system of government in which the premier would be elected rather than named by the king.
"The king has repeatedly complained that successive governments did not implement his reform vision. Why would we be confident that things would be different this time?" Bani Rsheid said.
"Jordan needs a roadmap that clearly delineates steps and a timeframe for democratic change."
A national dialogue committee, formed since the protests, has proposed new legislation on elections and parties, but its proposals have been criticised by the Islamists and conservatives as not ensuring a fair representation.
"The king showed strength in telling the Islamists and conservatives: no changes to the committee's outcome," said Labib Kamhawi, a founder of the newly-created National Reform Front, grouping opposition parties and trade unions.
"The king was weak and vague on major issues like corruption, freedom of speech, the role of the youth and the mechanisms to implement the promised parliamentary government. He did not say when and how."
In his speech, the Jordanian monarch warned that "no one in Jordan has a monopoly on reform or its promotion."
"We seek a state of democracy, pluralism and participation through political reforms ... away from the dictates of the street and the absence of the voice of reason," he said.
Bani Rsheid said the king "did not elaborate on the fight against corruption in a satisfactory manner."
"The measures against corruption are far from being effective. Nothing shows a real commitment to fighting corruption, while the scandal of Khaled Shahin, or 'Shahin-gate,' remains unresolved," he added.
Prominent businessman Shahin was last year sentenced with three others to three years in jail over graft payments as part of a $2.1 billion project, but he was allowed to leave for the United States for medical treatment in February.
Shahin was spotted in a London restaurant in April, causing an outcry in Jordan.
Stressing his "firm" fight against corruption, the king warned that dealing with it "on the basis of rumours and gossip ... mars Jordan's reputation both regionally and internationally," damaging investment prospects.
He also cautioned against "the deterioration of political and media discourse into one that aims to trigger hatred."
"We want a media that can carry the message of freedom and reform, optimise the accomplishments of our country, and protect national unity and the relationship among Jordanians," the king said.