Gear:
Shoes: This is really a personal decision. I had great experience with a lightweight hiking shoe. A couple others wore running shoes (including our fastest crosser at just over 7 hours).
Socks: Again, try out different types until you find what works for you. Only rule is NO COTTON. I used Smartwool brand ankle socks, medium thickness and they worked great for me. On the way back, I did have a small issue with the two littlest toes on my left foot cramming together a bit, so I'm going to experiment with toe socks for next year per Krista's advice. The pair of toe socks I bought were just too thin. Will try a thicker pair, or maybe go with a thin toesock liner under the Smartwool socks.
Gators: Everyone in our group uses gaiters from here:
https://dirtygirlgaiters.com/. They keep rocks, sand and dirt out of your shoes. I actually bought a different (seemingly more rugged and professional) pair of gaiters from REI and they weren't even close to these in terms of comfort and effeciency. Yes, most of the patterns are loud and obnoxious, but they are extremely effective and you can have fun with them if you want. In my opinion, they excelled because there is no elastic band around your calf, and the simple way they hook to your shoe. You don't notice that you're wearing them at all, which is one of the primary reasons they work so well.
Underwear: The Ex Officio recommendations in this thread were great. Had no problems whatsoever. Remember, you need to go synthetic here - no cotton!
Shorts: Had a pair of really light Columbia hiking shorts. No problems here either, but so long as there is no friction, the shorts should be no big deal. Some people wore light athletic shorts.
Shirt: Went with this shirt:
http://www.exofficio.com/products/details/mens-air-strip-long-sleeve-shirt. It worked incredibly well. Kept me warm enough with the sleeves rolled down that I didn't need a fleece to start the day. When it got hot, I simply rolled up the sleeves. The shirt is vented in the back and sides to help during the hot temps. It also has a collar that you can unbotton and unfold to keep the sun off your neck (though I had a wide brim hat and didn't need that feature). Seriously, I love this shirt. It kept me comfortable through a 60 degree temperature swing. (Yes, you'll want a jacket at 42 degrees, but you warm up fast enough once you start hiking. It was nice to not worry about carrying a fleece around all day.) Has two front pockets to store items for easy accessibility, and a velcro loop.
Hat: Okay, I looked a bit ridiculous, but I went with a
Tilley and loved it. Kept the sun off my face and neck all day. The band absorbs sweat, keeping it out of my eyes, and is vented at the top to release hot air. Durable enough to dip in the river to keep wet and cool. Has straps in case it gets windy.
Buff: Another great reccommendation from Krista. Wore it around my neck to keep me warm in the early morning. Later on when things got hot, I dipped it in cold water (at water stops or in the river) and wore it around either my neck or my forehead like a headband.
Trekking poles: An absolute must. Provided much needed stability and saved my knees on the way down, and helped a ton on the ascent (it's better to use four limbs to push yourself up the trail). Even across the floor of the canyon, where you don't need them, I found them handy for a couple reasons. First, they still help give you stability on an uneven terrain. Second, they keep your arms and hands elevated. My hands would swell during my long walks with my arms swinging at my side. That was never an issue on the hike because of the trekking poles. Mine were Leki brand.
Pack: This really depends on what kind of hike you are doing. I bought a Camelback pack with a three-liter lumbar water reservoir. I also bought an insulated hydration tube to keep the water outside the back-pack from getting hot.
Platypus and electrolyte tabs: I bought a one-liter platypus (flat plastic water container) along with Nuun brand electrolyte energy tablets. I would fill it at a water stop and clip it to my pack for easy accessibility. Restoring your electrolytes is a must.
GU Chomps: These are like large gummies and are good for energy (some have caffeine and some don't). I tried the GU energy gel and thought it was totally gross. Much preferred the chomps.
GPS Watch: I bought a Garmin Fenix 2 GPS watch on EBay for about $200. This was pretty much a waste of money. It lost the satellite several times in the canyon, and way overstated my estimated mileage. It was good for tracking the elevation though. There are much cheaper options here if you are interested in tracking elevation. Also, if you're hiking the Grand Canyon, the trail is well marked. You don't really need a GPS watch.
iPhone Case: Rather than bring a heavy case, I just brought my iPhone 6. But with all the rocks on the trail, and hiking along cliffs, I didn't want to run the risk of dropping my phone while taking it out for a picture. Remember, you are going to be sweaty and exhausted. So I picked up a
Beeline case. Clipped it to a beltloop and kept it in my front pocket. Worked perfectly!
Headlamp: Go cheap and small. I paid too much for mine and only ended up needing it for 15 minutes. Niles was spot on here. My headlamp was also bigger than necessary and you'll be carrying it 30x longer than you'll be wearing it. Every ounce counts.
And yes, Body Glide is your friend. Thanks to Chaos Commish and others for this pro tip.
Other gear was fairly standard: moleskin, first aid kit, compass, knife, sunscreen, chapstick, kleenex, moist towelettes, moisturizer, map, emergency blanket, etc.
Oh yeah, one other thing. For water purification, I picked up a
Sawyer Mini filter system. I didn't end up needing it, but you need to always have something in the Canyon because the water pipes go out from time to time or you may just run out between stations. I liked the Sawyer Mini because I could either screw it on to my platypus or cut it directly into my Camelback bladder system (I brought extra tubing for that purpose).
Again, most of the above just validates everything everyone offered in this thread. Thank you so much for the incredible advice. Please don't take offense if I didn't mention you by name. All of the advice and assistance was gold, and made a huge difference.