My two cents:
The ten essentials are great but they aren't a replacement for thoughtful preparation. You can't just have a "ten essentials" kit that you throw in your backpack for every trip and consider yourself to be prepared.
There is some good reading on wikipedia about the 10 essentials.
In general, thinking of systems instead of items is a good idea and the updated list is better than the classic list for that purpose. Going through the motions of carrying ten essential items is not the point. You need to thoughtfully prepare your systems for your trip based on the scenarios you will likely face. Hydration is a good example. For one trip you might need to carry a stove to be able to melt snow. For another trip you will need to carry extra water for the duration of the trip. For yet another trip it will be sufficient to carry a few water treatment tablets. It's a system, not an item.
There are also trips where additional essential items will be added such as avy gear in the winter or perhaps bear spray when travelling in grizzly bear country.
Here is the scenario that worries me the most when I am in the woods:
It's December or January and I'm hiking or snowshoeing alone in an area with very little traffic. I'm travelling on snow but it's currently raining and almost freezing. I have a gore-tex jacket and pants but all of my clothes are sweaty. I'm going downhill so I'm wearing most of my extra layers. I slip or post-hole and break a leg and can no longer move. I will need to wait until I am late and a search party is sent. That means I need to keep myself warm for many many hours. That's one of those situations where I would need many systems to work flawlessly and I'd need them all to be fairly robust (lots of extra layers, a good temporary shelter, food, candle and matches, etc.) After thinking that through, I normally also add a personal locator beacon to my list of essentials if there is a chance I will face that scenario.
One omission from both lists is a time keeping device. I always like to have a watch so that I can make decisions about progress based on fact instead of emotion. I consider it a part of my navigation system but it's easy to overlook if it's not called out.
Before thinking about the ten essentials, there are two other things to always do in the preparation phase. I feel that these preparation tasks should be added as Essential system #0.
1) Let someone know where you are going and when you will be back.
2) Walk through the trip in your mind. Study the maps. Know the distances and elevations and estimated times. Look at the route on Google Earth. Study a weather forecast.