The biggest challenge for me, as a developer was to convince higher authorities to agree upon a technology! 2 years into my career, I had to develop a Travel Portal in Struts 1. I was like, Not Struts again! It was Spring's blossom period and it was gaining traction. I suggested Spring, stating the advantages of Spring over Struts, but company was so adamant, they said we do not want new technology now! After a lot of face-palming and explaining, I finally made CEO realize it is not a new technology, but rather improved Struts framework(I know, but what can I do to make them accept?). And he agreed. So we had brain-storming session, POC session and after a month, we were into development of travel portal in Spring.
But I couldn't replicate this success everywhere. Because people are too comfortable to see outside. If I suggest we'll do X module in Y language, they will look at me like a fool. For me, this was, and still IS biggest challenge! I have couple more, but I guess this eats them for breakfast.