Is it normal for disc brakes to smell of burnt plastic? So, a new bike, Avid Elixir 5 180mm front and rear. My second ride was on a short dirt road, 340m denivelation.

I held the brakes (mainly the front) more than 50% of the time, as the road was too steep for my skills.

At the end of the descent, there was slight smell of burnt plastic, coming from the front calliper.

What has caused this and is it at reason for concern?

On another account, a couple of Tektro Augura smelled severely of burnt plastic at the middle of the descent (another bike, another rider). She finished the descent and has not seen ill effect since (well, the front braking is not as good as before, but no idea if this is since that happening or later).
Sounds like you may have "glazed" your pads and rotors. New brake pads require a break-in period before you subject them to hard long braking. 

http://www.bikeradar.com/us/gear/article/workshop-how-to-bed-in-disc-brake-pads-31337/

Even when you already have your disks broken in, long drag braking can cause them to overheat and fade. This leads to a kind of vicious cycle in which the brake is less effective and you start using it even more. 

Brakes are designed to cool themselves with the available airflow, even a few seconds every minute can make a difference. Of course, that's easier said than done when you feel like you are over your head. Braking effectively is a skill that takes practice. If you are dragging the brakes constantly to control your speed, you'll be better off coming to a complete stop and regrouping. 

Sometimes, you just need to walk stuff when you are over your head. 

http://www.singletracks.com/blog/mtb-training/how-to-brake-your-mountain-bike/