There isn't too much visual difference between Hongdae (홍대앞), Edae and Myeongdong (명동) to the first time tourist. The bigger brands are standard and brightly lit, the boutiques are small and narrow, the streets are bustling with tourists and locals looking for a special something to take home and the shop attendants are very skilled at convincing you to part with your money. But spend awhile in there and subtle differences present themselves.
You really get a sense of how much Korea has adapted itself for tourists when you step into Myeongdong and find yourself being greeted by an onslaught of Chinese, English and Japanese-speaking attendants (moreso when you pass several pairs of Chinese tourists buying masks in 500-pack bulk). It's particularly amusing going as someone who could speak all aforementioned foreign languages and not Korean but Myeongdong is, by far, the most international of all the three areas.
As soon as the sun goes down, the streets become a bustling maze of brightly lit shopfronts with all your famous makeup brands, dotted with night market stalls of street foods and sock stalls and full of foreign languages. You can definitely eat dinner just wandering from stall to stall and it's definitely worth going to for the atmosphere, if nothing else.
We didn't really go inside the large shopping centres in Myeongdong except for the Migilore department store. I recommend it for travellers looking for a bargain and clothes that are alright in quality. You may have to do a bit of digging and searching through all the stalls but you may find a pretty good bargain inside.
Seoul has a bit of a haggling culture so you can always try to ask for a bit of a discount in small boutiques but be prepared to come up against resistance - the shopkeepers are skilled at making you part with your money. Very skilled. Be careful to also inspect the quality of the clothes.
Supposedly, the cheapest clothes come from boutiques near university areas and near said university metro stations - we visited Ehwa Womans University (named so for a reason) and Hongdae. The price point isn't that cheap, though cheaper items tend to be better quality than its price equivalent in Sydney. We also caught Edae students preparing to give some Christmas performance involving...lumberjacks, Irish dancing and lots of eyemakeup. I'm not sure if I want to know.
Many of Hongdae's streets are pedestrian-only so you can just walk up and down, browsing stores. The area had a much younger, more indie vibe that really appealed to me and made me wish we had picked a guesthouse near the area instead. Hongdae seemed to house slightly 'younger', more cutting-edge clothes (and more menswear) whereas Edae clothes tended to be more boutique-like. Both have a whole array of interesting cafes and Hongdae boasts quite a few nightclubs - and that's just on surface-level.
Most of the store owners are open to a little haggling so always ask before buying. It can be really easy to be overwhelmed by all the choice so if you have the time, I'd recommend earmarking stores (carefully so you don't get lost) and coming back after doing a round in the area first. You'll find lots of the same items at varying prices so don't jump on the first store you see. Or build a rapport with the shop owner and try to haggle the price down.
Style Nanda reps handing out balloons. We didn't manage to visit the flagship store, unfortunately, but that's probably just as well or I would have spent all my money.
Only in Asia will you find a store non-ironically named Sexy Cookie. But that's part of its charm.