What's the real issue with immigration?

A little while back I made a blog post on my richardsonForPresident.com blog asking about immigration. If you don't want to follow that link I'll repeat myself

The other day I had the thought that I don't understand the issue of immigration. I learned about the great immigrant heritage of the United States of America and I find it difficult to understand what people are worried about. I know it must be something because immigration seems to be one of the main issues right now. So I'd like to hear from you and get some more information on what people's concerns are.

I will say that I have some thoughts on immigration from what little I do know. However, I will share those after I learn more and my opinions start to take shape based on the larger picture. So once again, I request your comments on immigration and your take on what some of the issues are that make it so difficult.

Since that blog will obviously be limited in perspectives that see it, I ask you the Opera community what you think. I'd like to hear the perspective of all types of people, conservatives, liberals, progressives, other Americans, non-Americans. Let me know what you think I'd love to hear your thoughts.