What a timely topic! I used to work for a very large enterprise company and there was a new VP that came in with an Agile mentality. But in large enterprise, there were more than a few obstacles. That VP joked that at one of his first meetings at our company, the purpose of the meeting was to decide a submit button color and then at the end they just decided to think on it and schedule another meeting to decide the color later! Aaaaargh. LOL, but not so funny if you're trying to do something. Today.
Agile is about working together, working quickly, and getting things done -- getting any unnecessary process functions out of the way to do so. A few of the things that could stand in the way are team members that are not all in one place (doable, but harder), managers that have a hard time letting go of "how we have always done things", and team members that are at different levels of experience (newbies won't be able to contribute as much or as fast as more experienced folks).
Looking forward to seeing the stories of real Agile marketers and implementers.