It's great when you have a cool chord progression, because your job is only to follow the lines. The difficult part comes when you have a not-so-obvious chord progression, say Am, Am, F, G. That's where the difficult part comes in. I acutally found out that sometimes it can be difficult to create great melodies and to make the chord progression really interesting. I personally would start out with a melody and then put some exotic soundig scales in it to make it sound more interesting
When I write a solo, I also like to invent new licks that probably hasn't been played before.