Flanking brings to mind three things to me.
The first is from the "real" world. I put that in quotes because this is just from my limited experience. I haven't been in a real fight since high school. I'm not an expert martial artist, but I've been taking kung fu for the last two years, practicing every day, and competing in (and sometimes winning) tournament events. I'm training for a full contact event now. Like a lot of kung fu, my style happens to focus on fighting more than one opponent at the same time in our forms, and sometimes we spar two on one. Flanking is usually how our two on one sparring starts, and if you're the one guy, you focus on using your footwork to move so that both opponents are on the same side. It's very dangerous to have one on either side. Certain stances and techniques can mitigate that danger, but the basic tactic is always to get them on the same side as soon as possible, which requires movement. Flanking does grant you a big "advantage" against someone IRL.
But D&D is a game, not a simulation, so the above is only of use to those who need some sort of assurance that the game is quasi-realistic, and it's just an anecdote, there are plenty of people with more experience and skill in real world fights (and not just sparring!) and I'm only mentioning it because it leads to my next two points. It's more important to consider what flanking does to the game at the table, and in my experience it's sort of the same thing. It encourages movement and thoughtful maneuvering.
The second thing that came to mind is the effect that flanking and attacks of opportunity had on D&D starting in 3rd edition. It was great at first because it forced characters to move, both to gain flanking or to get out of it. Up to then, fights in D&D at my table felt very static. Typically a fighter would rush up to a monster and attack it, and everyone seemed to stay in the same spot. The increased movement was new and fun and enhanced miniatures play, and it felt more cinematic, more real, and more fun all at the same time. AoO's were a bit clunky and had their own issue, but this thread isn't about them so I digress.
The third thing is that these tactical rules tended to cause issues in the long run because as characters leveled up, combat took too long. The problem wasn't in these rules themselves, it was that because of the design of powers in 4e, you couldn't take these options out of the game to speed things up if that's what you prefer, because it nullified so many powers that influenced movement. If you needed a faster game because of time constraints and/or number of players, or didn't want to use mini's, or just because that was your preference, you were out of luck.
So what I'm hoping to see in the DMG are optional tactical rules that encourage movement and maneuvering during combat. Movement is what it's about; watch any action movie and they usually aren't just standing there trading punches, the fight itself moves, sometimes all over the bar or even across city blocks; it's an exaggeration of a real fight where there is some movement, and that cinematic combat is what I like in my D&D. To me encouraging movement so that the combatants interact with the environment, and thoughtful maneuvering is the fun that flanking brings (and to a point AoO's and powers/spells that affect movement or move characters), so I'd like to see options that encourage movement and maneuvering, but it doesn't have to be flanking per se.