This paper is dealing with the problems concerning the improvement of regional classifications by using a composite index of economic development which encompasses four individual indicators: GDP/capita, labour productivity, FDIs and life expectancy. Our aim is to offer a better methodology and a multi-sided image on the regional development, from the perspective of two major factors of influence: Romania’s accession to EU and the recent economic crisis. A special attention should be paid to the hierarchical position of each influence factor. Depending on the specific factor mix, we try to draw conclusions regarding the relationship between economic resilience and vulnerability of each regional economy. The new approach based on the composite index’s computation has the advantage of providing a unique answer on problems such as unclear hierarchies or even contradictory results emerging from different classifications that use separate indicators. The study is covering the 2001-2012 period, divided into two sub-periods: 2001-2006 (pre-accession) and 2007-2012 (post-accession).