Expansion pro's and con's

Hello, I'm studying for an upcoming exam and I'd like your input for my study group.  The question we're discussing is:  what are the reasons for Rome's success in expanding its empire...and...what obstacles did it have to overcome?  

Your expertise would greatly assist us as we've been fighting about what should be included in an essay answer.  Some of the responses given are just nuts!  We need some wisdom.  Thanks