Margins and leading (line-height for those who prefer CSS) are two things that can be an afterthought when it comes to implementing a design. I know of plenty of times when I'd be integrating a site and give little thought to the line height or the margins in certain areas. But, time passes and I've learned that consideration to typography and things such as margins and leading actually do make a difference in the user's experience on the site. Consider the following four images:

Good Leading, Good Margins

Good Margins, Bad Leading

Good Leading, Bad Margins

All around terrible

Witchita State University conducted an experiment to find how leading and margin affect the ability to comprehend. The basic gist is that as the margins get wider a person is able to read less words per minute, but their comprehension level increases as the speed decreases. Read about the entire study here. And get those margins in there and space out those lines.