Boulder and Denver
Boulder has become a mecca for neopolitan pizza recently. Pizzeria Locale, Pizzeria Basta and Pizzeria Da Lupo are all excellent and would fit the bill for a casual lunch or dinner (Locale being my current personal favorite).
Other personal recommendations based on your criteria would be Zolo Grill (southwestern), Radda Trattoria (northern Italian), SALT (modern American with lots of local ingredients), Kitchen (ditto), Centro (upscale Mexican), Jax (New Orleans fishhouse) and Happy (upscale pan-Asian noodles, buns, and entrees). If you're looking for a sandwich for lunch, Dish Gourmet or Caffe (right next door to Pizzeria Locale) do great take-away sandwiches.
2525 Arapahoe Ave, Boulder, CO 80302
Jax Fish House
928 Pearl Street, Boulder, CO 80302
950 Pearl Street, Boulder, CO 80302
SALT - Boulder
1047 Pearl Street, Boulder, CO 80302
Jeans are accepted at Frasca, but it's such an exceptional restaurant w/ such exquisite service that dressing up a tad is more appropriate. The Flagstaff House is Boulder's enduring fine-dining restaurant, and they do have a dress code in the dining room -- not on the Terrace of summer cocktail hour. Arugula is first-rate; Tangerine, their bkfst & lunch place, just opened a few days ago and is very good.
Flagstaff House Restaurant
1138 Flagstaff Dr, Boulder, CO 80302
Honestly, Boulder is a small town, while we have an awesome restaurant scene, things just don't change much in 18 months. Oak at 14th, mentioned in the that thread as not yet open, opened and then closed because of a fire, and has not yet reopened. But that thread has been updated over the past 18 months with new additions, and I'm unaware of anything that should be taken off.
Also, FWIW, just about everything in Boulder is casual and relaxed. Even the fine dining options like Frasca, while still fine dining, don't require ties and such. Jeans is fine.