I think it is more of a lack of effort and good coaching than just bad personnel, but certainly the team lacks toughness.
The Raptors have always had a range of bad to terrible defense. We've probably been the worst defensive team of the decade.
i actually concur with the previous poster- its a personnel issue more than anything. lets be honest- bargnani, caledron, beli and turk dont play defense. aside from our euros- our star isnt exactly fond of the notion and jack who was heralded by colangelo as a defensive stalwart is a competent defender at best. i think wright is a solid defender but he doesnt have the strength to fend off physical wing players like melo in the post.
i thought grange's blog comments were incredibly revealing about the issue http://www.theglobeandmail.com/blogs/from-deep/failing-grade-for-calderon/article1369457/
I also think it's on your best player to lead the way. Look at how fast the Celtics improved their defense once KG came aboard. If your best player is committed to the defensive end and his teammates respect him they are going to also increase their effort.