Whole of my family still lives there, I left shortly after college at about 26 yrs old.
I could probably cover the negatives in a pretty short list:
1) housing is expensive. Simply due to density of population. While a standard 4 br house on 1/4 acre lot in a suburb can easily top $500k, 500k in Colorado will get you a very upscale house, and a bit more land. With high property prices comes high taxes too. Based on limited research I've done in considering move to SoCal....DC area probably comparible for the housing prices. Of course the further out you live, the more you can get for the money, but....(lead in to #2)
2) Traffic is horrible. Trade off for where to live. Short commutes (depending on where you need/want to go) usually come with higher prices. Metro is expanding further out so that helps.
3) Lots of people...many not so friendly/open as I've come across in other parts of the country to include SoCal. IMO put it down to lots of 'keeping up with the Jones' and putting out appearances instead of being authentic.
4) very easy to get 'washingtonized' or politicized as much of the news is so focused...limited 'local' coverage in my recollection.
1) lots of diversity.
2) lots to see and do in DC esp if you're not from that area.
3) beaches not too far, likewise for nature and mountains (hills really by CO standards! haha!) and woods, etc. just a few miles west/north.
4) lots of opportunity whether looking to job hop depending on your field, and lots of opportunities for vegan bodybuilding tips, training, probably even a vegan-vegetarian exclusive gym w/nutrition classes, cooking, etc..
I rarely go back. And when I do am quickly reminded why I left in the first place. Quality of life for me was better anywhere else. note: Colorado can spoil you in comparison to major parts of the country. Low cost of living, not real traffic issues (with exception of Denver during rush hours, no humidity, good temperate climate. Friendly people....but still, would consider living possibly in SoCal or Hawaii if just for a few years.....
Where abouts in DC area you thinking about?