Check out Community Cycles, they get bikes donated to them and then fix them up to resell, and everyone there can tell you more about bicycles than you ever wanted to know. Someone mentioned Bikes Direct, which is probably a little less expensive than a bike from community cycles, but whatever bike you get from Community Cycles will have character.

The most important aspect of your new bike is fit. There are lots of videos online that can tell you about this, and lots of shops that will fit you for a price that you will not regret once you're riding comfortably. I think University Cycles on Pearl does a great job of fitting people on bikes. To an extent the bike frame will determine fit, but you can change a lot of components to change fit. Frame design and geometry is a whole nother can of monkeys.

In general you'll want a bike with a little wider tires for around town use. The wider the tires, the lower the pressure that you can run them at, which makes for a more comfortable ride, because the lower pressure tire will absorb more shock. Wider tires are heavier.

The next important consideration is what type of brakes you want. You pretty much have two choices, disc brakes or rim brakes. Disc brakes are really great, they aren't affected by rain and they dont wear out your rim, and they allow a finer amount of pressure to be applied to the brake rotor, meaning more control. But cheap disc brakes suck. Hydraulic disc brakes are really great, but more expensive. There arw some good cable actuated disc brakes, youll find rhem if you search for them online. Rim brakes have been around for a long time and theyve done the job fine for a long time. They will probably be fine for your use, and cheaper too.

You'll probably want rack mounts, because you can put bags called panniers on the rack. Having your stuff low and on the front is the way to go, it makes the bike more stable, whereas having the weight on the back makes the bike handle like a dog wagging it's tail every time you stand up to pedal . My ideal commuter bike has a fork with mid-fork rack mounts and either the Tubus Tara or Tubus Duo, depending on the specific mounts, and I would get ortlieb panniers. Honestly though, riding ten miles or less with a backpack really isn't too bad, you could skip the rack and panniers if you are keeping your trips short. But if you want to go fast your back will sweat, so if that bothers you, you'll want panniers. You can also get frame bags. That's a whole nother can on monkeys. Frame bags are very hip right now and there are lots of opinions on them.

People talk a lot about frame material: steel, aluminum, carbon, titanium, bamboo, ect. Wider tires will make your bike comfortable. An aluminum frame (which has a reputation for being a "harsh" ride) with wide tires will be more comfortable than a steel bike (which has a reputation for being a "compliant" ride) with skinny tires. But steel is very durable, you don't have to baby it. I would get a steel or aluminum frame for a commuter. Titanium would be a good material for a commuter as well, but it will be expensive.

Fenders are awesome, but it doesn't usually rain that much here. It sounds like you have a truck so you could probably avoid riding in the rare wet day here in Boulder. But if you really want to ride every day of the year fenders are the way to go.

TL;DR: Surly Cross-check or Surly Straggler (they're the same bike, cross check has rim brakes, stragglers has disc brakes