What are you guys doing for layering when out hiking/ camping/brushbusting as far as pants go?  Polypros under bdu's?  Do you add an outer layer thats waterproof  or  wear a thin layer between polypros and bdu's?  I ask inbetween because it seems most waterproof pants don't seem very durable or tear resistant.
     For work I just wear cotton type pajama pants under jeans and that gets me by, but I am looking for a different system when out in the woods sitting,kneeling on cold wet surfaces while also being lightweight and manueverable.  Thanks .

Take a look at Arcteryx pants like the Sphinx, and Gen 2 combat pants they are VERY durable and water just rolls right off them. They use a tweave durastretch material that is very capable.

I wear my regular cotton long johns, since that is what I prefer and my poly blend BDU's.

I often have tall boots or gaitors to keep the bottoms of my pants dry, and for sitting and kneeling I pack in some reflectix as a pad.

That is for camping/hiking though. When I am hunting I usually have an outer layer on, and I worry a lot less about keeping my legs and ass dry because I keep warm moving and head home when I am done.

For something cost effective, track down some older ECWCS Gore-Tex pants.  For more "bushwhacking" I layer poly-pro (or light weight wool) long underwear with a NYCO type trousers and if weather sucks, the Gore-Tex trousers.  If your regular pants get wet and your ECWCS trousers are packed, you can just swap them out...lot of options depending on the weather conditions.

ROCK6

I wore these on a small trapline with some polypros under.  Wound up busting through overflow and was waist deep in a giant slushy that was a creek in summer.  Water didn't soak through it was 30 degrees out that day.
http://www.carhartt.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10101&storeId=10051&productId=156382&langId=-1

Not sure I would sit in the snow though. For that I would prob go with the goretex ECW shell

Carhartt pants with PJs(fleece, cotton, flannel, etc) underneath when I am walking, biking, or light hiking. I wear some sort of boot or shoes with them(Danner, Xtra tuf, loafers). I picked up some Swiss Mountain gaiters to keep any snow and wet off of the pants, as well as not feeding my pants to my bike.

If I am playing in the snow(ski, sled, screwing around) I am wearing snow pants with a water proof membrane. Current pair is Burton with Gore Tex. Staying dry is important for me to enjoy being outdoors.

Surprised so many of u wear a cotton baselayer/long johns.  My old motto is "cotton kills" when u sweat and it than freezes.  

Merino wool baselayers has been the best tip anyone ever gave me.

If you're involved in static activity or it's very short term, cotton is okay.  I still advocate for synthetic or Merino wool.  My (actually OIF/OEF RFI issued) Polartec silk-weight long underwear are the best I've used for high-level activities but I often stick with Merino when I shift from sedentary to active and always part of my sleep system during colder seasons.  

ROCK6

Depends on what kind of brush you're busting and whether you need maximum protection from it.  If you do, Filson makes waterproof canvas that will allow you to go through anything nature can throw at you.  Greenbriar, blackberry, etc.  I've been in it so thick I could barely move and had to protect my face with leather gloves as I went, nothing got through it.

The cotton long johns work fine for me. Im not a sweater, not even in summertime. Hell, in the winter I could run a marathon with long johns on and probably not sweat enough to be an issue. Im a Florida native and I hate to be in the cold and snow.

It only kills if you aren't prepared to deal with it if it gets wet.

I keep extra base layers inside a ziplock inside my pack liner.

If I go in the drink just make a fire and change clothes.

I fell in the river last Christmas eve actually while hunting, fully submerged. I got out, stripped down and rung out, lots of daylight left so I started walking back to my truck, the walk kept me warm, took about 45 minutes. It totally sucked, but I was able to deal with it.

If I was on an overnight and that happened I would be setting up my tarp as a heat reflector, getting naked with a fire going and drying all my gear while I hung out in my extra base layer, maybe down around zero that would suck, but for most of the temps we get here I would probably make it.

Ive been doing well with wearing nylon "hiking" pants (treated with dwr) with a mid weight merino bottom, I always keep packable rain gear in the pack.

AGREE 100%
I get cold wearing cotton longjohns. Get sweaty and forget it!
I wore them up until the  mid-late 80's when I found out about  poly-pro and dou-fold.
There have been a couple times when I've wore cotton tops since then and froze.
Hard to believe I used to rely on them.
I try not to work up a sweat when out in the cold, and I layer.
Sometimes your gonna get sweaty and without the ability to wick moisture from your body, your gonna get get in trouble.
Modern synthetic fabrics have come a LONG way in the last 10 -12 years even.
Much easier to stay warm than when I was younger.

I wear polar fleece grid level 2 tops and bottoms with bdu's alot.
If it is really cold,(teens to single digits)and I'm not gonna be real active, (fishing)   I wear a 4 way stretch polarfleece union suit over silk weights

This.

I'm comfortable down to 0° if I'm actively moving or resting for a short(er) period of time. Like above, I wear a synthetic base layer with REI water resistant (damn near waterproof in my experience...) "cool weather" hiking pants. They have a very thin layer of fleece that's perfect for winter hiking. Add some Black Diamond gaiters, Smart Wool socks and a leather GORE-TEX boot of your choice and your good to go.

Down to about -15F or so I wear Gen III ECWCS silkweight polypro or Stoic 150-weight merino under Cabela's breathable rain pants. If it's colder than that I wear Cabela's Thinsulate/DryPlus snow pants over the polypro or wool, sometimes over both the polypro & wool. I've found the Cabela's brand stuff to be plenty tough an reasonably priced.