I'm a big believer in "just a bike". Too many people go for a bike that has unique handling characteristics, a really cool custom paint job, super relaxed (or twitchy) geometry, or an unusual design (like the Cannondale lefty fork). Unless you are a builder or manufacturer, gravel bikes are TOOLS to be ridden hard, occasionally put a way damp, and a fancy paint job is going to get trashed - or you aren't riding hard enough! Just a bike.
Any old cyclocross bike will do quite well, but most have limits on tire width, have a tall bottom bracket (for clearance, rim brakes and the kind of handling CX requires), and few brazeons.
Picking out your requirements (in order of priority):
The big thing is to go ride at least a handful of bikes. You probably won't be able to ride gravel, but you can often sneak off onto into a park and onto a soccer field or baseball diamond to at least get a feel for 'off pavement'. Ride the bike, ride another bike, ride another bike. One of them will sing to you. PAY ATTENTION!!!
Granted, this is *my* priority list. Your choices may be in a different order or have different criteria. These will give you an idea of what to think about. I am working from the assumption that most people will be on road bikes now. If all you've ever ridden is mountain, the information is still about right, but I'd steer you toward a 29er with 35-38mm tires, flat bars and no suspension.
Where are you going to ride?
If your plan is to just ride on the Wisconsin 'gravel' state trails, you will approach several of these choices differently. On the other hand, if you have dreams of the Dirty Kanza, Land Rush 100, or Rebecca's Private Idaho, you'll want to select on the 'beefier' side of things.
Price point.
Start out by figuring out what you can afford to spend. This is probably the most important criteria, since it will set out components choices, frame materials and other choices. Within a given price point (like $1000, $1500, etc), most bikes will have a very similar group of components. The brands may change, but the quality and weight are very similar. It'll be the minor details that help with your choices.
Brakes
Price point is really going to drive your frame material choice, but within a range, you'll probably have the option of disc or rim brakes. After riding both rim brakes and disc brakes this season, often on back-back days, my jury is in. I have bought my last rim brake bike. Disc stops better, allows more rim choice, better modulation of power, and they stop better.
Tires
Pick your tires based on where you ride. Maybe you'll even want a couple of sets. I have 28mm tires for the road only, and 38mm tires for gravel, plus a set or two in between. If you are spending 1500 and up for a bike, you can afford an extra set of tires for smooth cinder trails and the road if it matters. Otherwise, go with as wide as you can cleanly fit in your frame, and look for something moderately smooth in the middle with knobs out on the edge. Something like a Clement MSO or Challenge Gravel Grinder. And if you can afford them, I'm very partial to Challenge Latex inner tubes. Thinner, lighter, and more puncture resistant.
Wheels
The late Steve Hed designed possibly the ultimate gravel rim - the HED Belgium C2+. The wide rim gives you extra volume, a better ride, lower rolling resistance, and probably folds your laundry while you sleep. In any case, I firmly believe they'll improve the ride, comfort and handling of any bike.
Frame material
Everybody seems to have a preference. I've heard all the 'steel is real', 'buzzy aluminum' and 'harsh carbon' comments, Bah. Steel rusts, aluminum is inexpensive and can be formed to make laterally stiff, vertically compliant frames, and carbon can be tuned to perfection. At lower price points, steel or aluminum dominate, at the upper reaches, expect carbon or titanium. Ti is a great material, but is difficult to form, and expensive. Any buzz from aluminum can be dampened out with good bar tape. A purpose designed 'gravel' frame will have a longer stay and more relaxed fork, which will also smooth out the ride. And yes, I'm in love with my Foundry Auger, and highly recommend it, but there are plenty of other good rides out there.
Drivetrain
At a given price point, you'll get what you get, and unless your price point is <$500, you'll get pretty good. BUT - look for, or talk to your LBS about getting 'cyclocross gearing' or even more custom. 36x46 or 34x44 front rings will give you plenty of top end gears and still allow for a low-low without resorting to that hideous combo known as a triple. In fact, it is possible to set up Shimano Ultegra 10 with an older Shimano XTR 9 speed derailleur and use an 11-36 rear cluster, which will give you a 1:1 ratio on steep climbs. I've only walked once this year, and have climbed numerous 14%+ grades.
Cockpit
Stay with Aluminum stems and bars. Period. Carbon bars are a lot better than they used to be, but they still should be regarded as 'one bounce', and may snap at the worst possible moment after that. I'm really partial to Ritchey WCS stems, tape, and Salsa Cowbell bars. There are any number of other good brands on the market, this happens to be my preference.
Accessories
Bags - start with a seat bag. Banjo Brothers and Revelator both make great bags that expand and provide some level of waterproofing. Add a frame bag for really long rides, and maybe a Bento box if you really want to. For rides under 100 miles, I find that I can fit everything in my tail pack, jersey pockets. Your mileage may vary, of course.
Cages - you want something secure. I'm sold on the Arundel Sport cages. Cheap, light, idiotproof.
Lights - lots of good choices out there. I'm picking up a set of Seca 1700 Enduro. Bright, long life, and easy to mount. Did I mention bright?
And that's a short list of picking out your own gravel bike. See you next spring at the Miesville 56 or maybe the ride formerly known as Almanzo.
~mj
