Imperfecto and indefinido.

Question

¡Fue tan interesante!I was sure that this would have been ¡Era tan interesante! Because I thought that the imperfect was used when describing something?

Answer

Have a look at this video which explains why you use fue rather than era. https://youtu.be/VKjPJao-UKw

Answer

Hola Peter
If we use the Indefinido fue in this sentence this is because we see this event as something that is completed/it had an end. 
If we'd used era, then the focus is on describing that event. 
Here is a Kwiziq lesson that will help with the understanding of these tenses:
Using ser in El Indefinido vs El Imperfecto
Saludos
Inma