In reply to john arran:
Unless you are sprinting or are running into a fair old breeze, the decelleration due to air resistance is negligable. You can see this if you cycle a bike up to about 10mph on the flat and then coast; it takes a long time to stop and even then, there is significant decelleration due to the tyres on the road surface. Force of the air on you increases with the square of the movement speed so while you might think "woah, this is some resistance" on sticking your head out of a car window at 50mph, a typical runner at 10mph is going to feel 25 times less resistance.
The angled treadmill is harder to run on because when you left your trailing leg, you have to lift it higher than you would if the surface where level because the back end of the treadmill is at a lower altitude than the front end. BUT, you are not gaining any altitude, as your foot travels from touching the ground back under you, it also falls in altitude, this means that you don't apply the force through that leg necessary to raise the rest of your body above the waist. So while running on a slanted treadmill is harder than running on the flat, it is easier than running up a real hill. (this assumes you precisely match treadmill speed at all times, if you run slightly faster then slightly slower by turns then you will be effectively gaining and then losing altitude with your trunk so it will feel a bit closer to a genuine uphill run).
Loads of things will make running outside more energy intensive, including:
lumps and bumps in land surface, kerbs, etc.
muscle response to keep you on track when your foot lands on uneven ground.
correction against wind buffeting from the sides.
extra effort if there is a strong headwind.
Energy to heat your body if you are running on a particularly cold day.
As you increase your speed on the treadmill, your grounded foot is travelling back faster so you have less time to bring the other foot forward for the next stride. If you want to stay above the treadmill (and not end up implanted in the wall behind you!), you have two options:
1) Stay in the air longer to give yourself more time for your trailing leg to make the passage through the air to being in front of you again.
2) Move your trailing leg through the air at a higher velocity so it goes from behind you to in front of you in less time.
For 1), aside from cheating, the only way to stay in the air longer is to bounce higher from every stride, this means more upwards acceleration which in turn means more energy to the muscles that contract and expand to propel you upwards against gravity.
For 2), If you bring your trailing leg forward at a higher velocity, your muscles have to use extra energy to accelerate the leg forwards just after you lift it up off the ground and extra energy to decellerate the leg to stationary (relative to you) just before it hits the ground again.
On a personal note. Running sucks!
Seriously, it is incredibly inefficient. I challenge anyone to spend 4 minutes gently freewheeling down a slight slope in the countryside and then honestly tell me that they preferred their experience running down it!