I would recommend a good pair of walking/running shoes, something that has good arch support. I, personally, would also stay away from sandals and flipflops, particularly when in the exhibit hall-- too many people and the likelihood that your toes might get stepped on... in other words: close-toed shoes. Also, boots are just too heavy, remember you're doing a lot of walking and standing.
As someone who has apparently been 70 years old forever as far as style is concerned, I have exclusively worn shoes like this since I was about 14:
Comfortable for endless walking OR playing shuffleboard at the retirement home!
OK, I have to comment on these shoes.
Yes, the price is up there, but let me say this: I have NEVER worn a more comfortable pair of shoes, and I've run marathons in good running shoes. These shoes are durable. I have a couple pairs and they are all I wear to work DAILY for the past couple years. NO regrets.
I would suggest at least trying on a pair before you dismiss the due to cost. Since they are so durable, the cost is offset by the long life of the shoe. I don't work for SAS, but seriously, they are great shoes.
Make sure you do some research and pick a shoe that is rated better for walking on CONCRETE. Some shoes may be comfortable if you are on carpet all day, but if you spend 8 hours on concrete, they are useless. If you are just now getting shoes, start wearing them now so they will be broken in by the time you get to Gen Con (There are some brands like Merrill and Mephisto that have zero break in. You pay a premium price for them, but your feet will thank you). You may be able to change shoes, but you can't change your feet. If your feet are hurting by midday, it will make for a miserable con. For added comfort, hit up the Dr. Scholls display at Walmart and grab you some orthotic inserts. Also, bring a change of socks and swap them out halfway through your day. Just throw out the other pair if you don't want to lug around stinky foot wraps all day.
Walking shoes (and what aaronmlopez said about shoes rated for walking on concrete!) with insoles. I have to have ones with ankle support to boot. I'm seeing a podiatrist in 10 days, will see what I end up with after that.
Make sure you do NOT wear NEW shoes or your feet will hate you!
I also agree about Dr. Scholls, I am totally regretting not having mine anymore!