Given MMM's location, I understand why CO is popular but Austin and places like NC/Georgia are mentioned more than the Pacific NW? Really? :)
I've lived all over the US and been to almost every state, and I think the Pac NW and CO are easily at the top of the list. If you hate snow, then you skip CO and if you hate occasional light rain then you skip Oregon/Washington. Seattle is expensive, but many locations away from the metro are fine and have fabulous weather, amazing access to nature, and mild weather all year long.
Some benefits include:
1) It is amazingly gorgeous all year long. No periods where everything dies due to lack of rain like some states.
2) Mild winters. Varies a bit by exactly where you live but it rarely gets below freezing. I don't think it has ever been below 0 in Seattle. While the east coast was hammered by snow storms this winter our "rough" winter was 2 stretches where it got into the high teens for a few days.
3) AMAZING summers. Rarely gets hot, and if it does it won't be muggy. The last few summers have been months of 75 to 84 degree weather with clear skies and angels singing in the background.
4) Spring/Fall also gorgeous, just with a bit more rain than summer :)
5) 293492342 amazing places to go hiking, skiing, go to the coast, whatever.
6) Mild climate means minimal heating/cooling bills. I think we spend 20$ a month on average on electric, and that's with being super lazy about turning things off, leaving computers and Tivo on 24/7, etc.
7) Great access to fresh/organic vegetables and fruit. Cherries here do NOT taste like cherries that you get in parts of the country that have to ship them in. I thought I was nostalgic until we moved back and suddenly the cherries were 5x as yummy like I remembered
8) Awesome seafood if you live near the coast, especially the Washington coast.
9) Neighborhoods where you can walk/bike everywhere are a lot easier to find and bike paths are valued and available in many locations.
10) Average person seems more educated than some other places I've lived.
It isn't perfect, but having lived on the east coast, the midwest, CA, various areas in the middle of the country (TX, OK, MS), etc it is a really awesome place to live.