Question: who are you designing adventures for?
Your own group, whose class make-up, strengths, weaknesses, and so-on you know like the back of your hand? Or some other group..? The five-PC archetype, all roles present and correct, sir, yes sir, and a spare which could be just about anything (but which the DMG recommends being a Defender).
If you're a professional designer, the answer is obviously the second one. You can't afford to do anything other than write to the archetype because the game is designed that way. But if you're the former, the point of writing your own adventures is arguably that you can put together challenges which your own group of players are particularly suited for, or which they specifically enjoy. The problem is, it might just cause you more problems.
Fast forward two years to the point where 4E D&D is starting to groan under the weight of accumulated splat books. One of your players builds a character akin to Eagle-Eye Action Men (tm) from the 80's (you all remember those right?) He enters a room. His slanty eyes look left. His slanty eyes look right. Hidden doors, crouching monsters, meticulously laid ambushes, weapon caches, escape routes, faded glyphs, tracks, buried treasure... suddenly they're all lit up like a Christmas tree. Malcolm 'Eagle-Eye' McSplat steps back, content in the knowledge of a job well-done, and his mates surge forward. Carnage ensues. The room is laid waste (and this is 4E remember; this room is *big*). XP gushes like blood from an artery.
And so it should, right? He's crunched the numbers, bought the supplements, spotted the synnergies. He's the type of player who gets at least half of his fun when the game isn't even running, in those in-between times when he's sitting amidst a ring of open books like a summoning circle conjuring up a tricked-out demon PC from the depths of DM hell (that's like the 509th layer of the Abyss or something).
But as his DM, it's entirely within the realms of possibility that you might want to start challenging his character a bit, so you start raising Perception DC's to a level at which he could conceivably fail, a level which is off the curve for a 'normal' character of his experience, but which makes things much more interesting. In doing so, you make the adventure a bit less predictable, but you make your player a bit more grumpy, because suddenly Malcolm's tenth-decimal-place build is for naught. Suddenly he doesn't know immediately that the room is trapped, orhow to disarm it. Suddenly, maybe, the rest of the party start to matter.
Today, right now, 4E is still in nappies. Straight out of the core, and with a few sneaky little errata applied, it's very difficult for a min-maxer to exploit a sick combination of powers, feats, and skills like this. You won't find a Fighter whose AC is impenetrable even when he's trussed up like a joint of meat and nailed to a bullseye, or a Sorcerer whose Invisibility is so much Greater than everyone else's that she is effectively immune to combat... but in a year or two, you will. No game as complex as D&D can be immune to this effect, and if the rumoured changes to the GSL go ahead and the door opens to a flood of third-party releases, it will happen that much more quickly. Then the question of who you're designing for will get very real.
As a DM you want to challenge your players. You want to make them think, squirm, maybe even make them sweat a little bit. A game which doesn't challenge the players is no fun... you may as well just build a 30th level PC at the start and cut out the middle-man. But at the same time you can hardly design an adventure that effectively bypasses their strengths and zeroes straight in on their weaknesses, because they will never feel empowered by their build decisions and will just get bored with having their backsides handed to them every Saturday.
Min-maxers aren't the only 'problem' (and I don't use the term pejoratively: as far as I'm concerned, you take your fun where you can get it). You also can't ignore the eccentricities of your group. Got a party of five goat-riding halfling paladins? It sounds broken on paper (hell it sounds broken whichever way you look at it), but your group thinks it'll be fun, and heck, you think it'll be fun as well. Forget for the moment that five Defenders are going to die of old age before completing enough encounters to reach 2nd level... it's like, five halflings, man! Riding into combat on billy-goats! Hilarity will no doubt ensue, but the DM, if he harbours any expectation of the campaign moving forward at all, will have a lot of work on his hands to convert -- or write from scratch -- adventures which will challenge this group and provide reasonable opportunities for heroism and success. In other words, he has no choice but to break the system to support his players. 4E's codification of party roles makes 'sub-optimal' parties more obvious from the outset, but doesn't make them any less likely to appear. Again, that's just the nature of the game.
I guess my point, if I even have one, is that the question of who you're designing for can be difficult. As a DM I like to write adventures that play by the book, and perhaps could be picked up by another DM, someone I don't even know, and played as-is for his own group. The proliferation of networking opportunities that the internet has given us to share our work has only entrenched this position. But as a DM I also like saying 'yes' to my players, and I certainly don't like forcing players to take characters that they might not be interested in, simply because there's a hole that needs filling in the group. It's a difficult balancing act, and it pushes me towards taking pre-published adventures and bending them to fit, for the simple reason that I can't stand the idea of writing something from scratch that can't be used out-of-the-box by other DM's. It's a philosophical road-block in my head.
And I'm far too old and set in my ways to change now.