These threads really irritate me due to the collective and abundant amnesia a lot of parents have. I've seen numerous threads in my years on Mumsnet of people having a good old vent about their kids - hell, there is a fucking blogger out there who makes an absolute killing with publicity by writing about the very negative connotations having (biological) children can have upon adults. Parents lap that shit up because it all speaks to us, parents and step-parents alike, over the trials and tribulations of parenthood. Fucks sake, nobody would bat an eyelid if this thread was titled "What you really think when your 3 y/o shits the floor" 
The moment you add the word STEP into that equation, everyone starts this "OMG HOW DARE YOU" complex that entirely perpetuates this stigma over step-parents and their entirely normal, appropriate reactions to things. Just because they haven't biologically provided the genetic make up of that child, doesn't mean they cease to have an opinion, especially when that child may live with them FT or PT. Everyone gets on their high horse about it without really considering how THEY would feel in that position, nor about the numerous times they've probably made unsavoury comments about another child either within their care (a friend of DC) or at school playgrounds.
The fact is, as a stepparent you DO have to control what and when you say things. There can be a variety of reasons for this, but usually it's because threads like this make it abundantly clear that THEY ARE NOT THE PARENT HOW DARE YOU and that they should be seen, not heard, and effectively sit there taking whatever so long as they don't express negativity. As a step parent, even if you are supported by your DP/DH, you still have to factor in the birth mothers POV and work hard to ensure you are at all times, safe in the eyes of your DSC, but without overstepping that (usually invisible and unspoken) boundary. It's a fucking nightmare knowing where that starts and ends, so for a lot of stepparents, it's easier to keep strong emotions (and this includes love, pride and affection) inside so not to offend birth parents/birth relatives, or seem like you're trying to "play Mum/Dad" - it's a horrible fine line which many people find hard - mainly because the majority of step parents I've met absolutely adore their stepchildren and find it hard to express how much they mean to them without overstepping the mark.
So yes - in the same way I would support threads that allow parents to "vent" the difficult aspects of biological parenting, I'd support that which allowed step parents the chance to release - this demographic is one of the most stressed positions of authority within a 'family' unit and is often the cite of major depression, anxiety and illness. It's unfair to allow "biological" parents this concession and kindness in venting, but to discredit and suggest if a step parent does this, they are somehow inhumane or contributing to developmental problems and/or happiness within a child. It's bullshit - it's life, people moan, people love, people care, people get angry. And it's worth noting for every instance of a "wicked stepmother" there are thousands of women out there who take on the role of being mothers without thanks, gratitude, kindness and respect - we are often the outcasts, the ones who dress our SC, feed them, play with them, do homework with them, educate them, love them, are fiercely protective over them but do so in the knowledge that to the eyes of many, we are just stepparents - we don't get to go to parents evening, school plays, doctors and hospital appointments. Many of us love our stepchildren dearly, and always feel that sinking feeling that we cannot attend these types of things, especially when we've helped contribute to it (sewing school clothes, reading lines of plays, going through homework). For some stepparents, they take the role of the ineffective, uncaring, absent biological parent.
So please, allow us the same concessions - many of us work hard to raise your children because we love them.