As far as i'm aware, if a map compliments any non-league standard class it's unbalanced and will never be accepted on that scene. That would atleast explain why competitive players have no love for A/D, a mode that justifies the use of Spy's and Engineers. 5 cp maps also generally do not cater well for the engineer, what with the dynamic battlefront.
Basically if they're confronted more than a couple times with either being outnumbered or out classed, they bitch and moan.
I find the competitive scene just makes things difficult for themselves. Especially when it comes to the 6vs6 setup they love so much. The game was designed in mind, by countering and mixing unit tactics. This just isn't possible with such a limited team of players. 6vs6 forces them into a scenario a map would likely not have been designed for, and as such causes them to resort to the most durable/dextrous classes available in the game.
By removing certain class elements from the game you remove the need for certain map design features. Such as already discussed, the requirement for (atleast one) decent engi spot. I just find this limits the experiences a map offers players and strips TF2 down to the barest minimum it has to offer any individual.
I want to offer people surprises and experiences. Not the same old dish each night, where you get killed by a demo, double soldier-medic combo (If you cry about being killed by the same sentry farm in pub play, then go running back to the comp scene, you're completely missing the point of TF2). This is my motivation to avoid bothering with the competitive scene. Even to this day i find new locations and tactics being employed by players on the stock maps, and if competitive players can't find a server with a decent amount of teamwork or mic users, then they just arn't looking hard enough. It's absurd to say pubbers never have mics, it would appear i'm not the only one who has a mic in this thread either. I just think i have better things to do with my time than appease self rightious, pious individuals.
Like have a wank.
(it should be noted that grazr has no ill feelings for the competitive scene, but facts are facts; and with every arguement posed in defence of the actions of competitive players and their demands, these are completely blown away by redundancy to the mass public crowd; and although this is a boring and repetative statement, that does not stop it from being true).
At the risk of sounding spiteful, if the competitive scene wants maps designed in a certain way then they can pick up Hammer and do it themselves, and stop crying about and abusing the 'experts' so much.