I think we all agree on two things: 1) running outside is fun, and 2) running on a dreadmill is not.

So if weather, darkness or whatever prevents you from running outside and you must do your run, then you pretty much need to set your mind on getting it done on a treadmill.  Just accept the facts that it's going to be dull, you're going to sweat more and you're probably not going to go as fast as you do outside.  The goal is to do your run, and if achieving your goal means having to do it on a treadmill then that's OK.

@Elite5981: My theory is that treadmill workouts can be more difficult because you're forced to run a constant speed. When I review my Garmin logs after an outdoor run, I notice that my pace varies slightly throughout my run. Not having the ability to make those micro-adjustments can force you to run faster than you want to at some moments, and slower than you want to at others. As a result, I find the unrelenting need to run at a single speed on a treadmill to be very exhausting. (That and I tend to overheat -- a lot!)

Don

I, personally, HATE, HATE, HATE the treadmill.  I have always trained outdoors.  I have hundreds of miles outdoors on different types of surfaces including trail races.  I only joined the gym and started using the treadmill because it is no longer safe to train outdoors in the dark without my dog and she can't handle my distance or speed.  With that said this is what I have found:

Cons:

I am slower on the treadmill than outside.  I do not know why.

I feel less stable on the soft bouncing surface of the treadmill and find I feel like I am going to fall off.

Dang, it is hot in the gym and I sweat much more!

I find the guy or gal pounding on the treadmill beside me very distracting.

Pros:

I can use the treadmill regardless of the weather and time of day.

I can vary my speed to vary my workout.

I can vary my incline which I cannot do outside.

Hey, I can watch TV while I workout.  LOL

Bottom line...I am faster and cover more distance outside in the wind and weather and sunshine.  I like it better.  But the treadmill is a necessary evil for safety reasons.  I cannot wait till spring!

I'm really suprised that so many people responding to this post say they run faster outside.  I run faster, and hold my speed longer on the treadmill when I leave it on zero incline.  However, because it is so much easier for me to run on the treadmill at zero incline than to run outdoors, training with zero incline has not adequately prepared me for running road races on hills.  I would have great treadmill runs and then when I'm running a 5K or 10K on the road I would die on the hills, ending up walking part of the race.

I've recently begun to agressively use the treadmill incline function, and I can't run nearly as fast on the treadmill when doing this.  I haven't run a road race since changing to this training technique, so I don't yet know if it will improve my road race times.  I may need to rest more between training runs to give my muscles time to recover and build up, but I'm hoping this will make the difference eventually.