"I am talking about not just reforms but credible ones. Bringing the benefits of the accession process to the people throughout the process rather than at the end," Füle told journalists at a Western Balkans conference in Vienna.
"Enlargement is not just a ticking-of-the-boxes exercise, but delivering on a track record," he said.
Füle's comments came just two weeks after European parliament elections saw anti-EU parties hostile to further enlargement scored major victories in several member states.
At a time of increasing reluctance to accept new members into the European Union, Füle hailed enlargement as the EU's "most powerful instrument" to bring about reform in aspiring countries.
The commissioner also spoke amid rising concerns that countries have not been adequately prepared for membership despite reforms made to join the EU.
Failure to undertake deep economic reforms are seen as a major reason why Croatia, the second former Yugoslav nation to join the EU, has seen its economy stagnate since joining last year.
Bosnia's efforts to join the EU suffered a setback earlier this year when Brussels cut back aid after the nation failed to remove laws that restricted the rights of minorities to hold senior political offices.
Meanwhile Serbia, which began EU membership talks in January and hopes to join the bloc by 2020, called it a "national priority" and appealed for the process to be completed swiftly.
"Serbia is committed to membership in the EU. We think that this work should be done as soon as possible," Foreign Minister Ivica Dacic told journalists.
He also called for continued talks with Kosovo, although Belgrade still refuses to recognise the independence of its former province.
"We should support the continuation of the dialogue with Pristina and the reestablishment and maintenance of stability and peace," he added.