I am starting my FNP program and I went to the ED as soon as I could for precisely the reasons listed here. I really would not want to work anywhere else in the hospital and think it is very good preparation for being a FNP. On a day-to-day basis the majority of ED patients are ambulatory and at an acuity similar to primary care settings. Of course you will also see much higher acuity patients daily, which is also excellent experience. In the ED you will also do a lot of teaching to both patients and their families. Additionally you will care for patients across the life span.
Probably the best thing I can think of about the ED in relation to a future FNP career is that the majority of patients require you to really use your assessment skills to listen to the patient, look, feel, auscultate, etc. Is a stroke or Bell's palsy; too many tacos or a MI, menstrual cramping or an ectopic pregnancy? You get the idea.
From my experience, in the ED you also work with the MDs more closely. They are always there which gives you as nurse an advantage to learn and be part of the diagnostic process.
Before jumping into the ED as a new grad do some homework. Does the ED have a solid preceptorship program? I would suggest three months under a preceptor as a minimum. Do you multitask well? Is the hospital committed to training and education? The ED where I work has a great bunch of nurses who really work as a team. In contrast, the ED at another hospital in my area has a reputation as "you're on your own" type of place to work.