The 15 experts on Eco-towns Challenge Panel, set up to examine and help the bids, said there was "much to admire" in the shortlisted towns.
But it warned many lacked ambition and innovation in some areas, while several - such as Curborough in Staffordshire and New Marston in Bedfordshire - were conventional developments.
More needed to be done to make the "eco" part of the scheme embedded in the development in those sites, not just an add-on, the panel said.
The 15 potential schemes for low-carbon, environmentally-friendly towns will be whittled down to up to 10 successful bids later in the year.
The Eco-towns Challenge team, which includes experts from areas including planning, design, transport and sustainability, was asked by the Government to review all the proposals and speak to developers.
They came up with a series of general recommendations for the developers to look at, such as what the town would provide for teenagers, how it could deal with the impact of climate change in the area and how public services would be provided.
The panel, which will not select or recommend which towns are selected, also came up with recommendations and areas each individual proposal needed to work on.
The team said transport could prove a particular problem for many of the rural sites, with concerns about how developers will change residents’ behaviour from a car culture to more sustainable alternatives.
Several of the proposals, including Rossington in South Yorkshire and North East Elsenham, Essex, need to show they are not reliant on nearby airports for jobs - as that would not fit with the environmental criteria of the towns, a report by the panel said.