I ought to start by saying that I may in fact be a dick.
Having gotten that out of the way, I often come across the sentiment that "judging others" is bad, and I have a hypothesis as to why many feel such an opinion has merit, but I find it decidedly hypocritical. People judge others all the time, what seems to throw people off is the context. I have found (this is anecdotal of course, ymmv) that few people have difficulty judging criminals, or people who cut them off while driving, or people who are rude to them. However were someone to turn the judging eye upon such folks, they are often the first to say, "You can't judge me!" or call you a condescending "bleep". Why is it fine for some people to be judged, but not others? Hypocrisy more often than not is the reason. What remains to be answered, though, is why the idea of judging itself is held to be problematic.
I can think of a number of reasons, and the two which come to the fore are relativism (and its stepchild, individualism) and the influence of Christian ethics. To explore the first in its entirety could (and has) fill several volumes of texts, but I will discuss it in short. The individualist perspective, that ones opinion is as valid as the next is not necessarily a bad thing, and forms the basis for many constitutions and charters. However it is also often untrue, the opinion of an expert in any given situation is stronger than the opinion of Joe Everyman on the same subject. Why do people seek out specialized professions for their needs (be it education, auto-repair, law, medical aid, IT, etc.) if everyone's opinions or knowledge base is equal? It could be argued that knowledge or skill is a separate category from opinion, but opinion is simply how one expresses their perspective, itself informed by their knowledge or skill set. The fact is that people who are more knowledgeable are recognized as being the people to ask for advice or services, and so their opinions are given more weight than others. Clearly then, not all opinions are equal.
Standards then, are the other part of the individualist dilemma. What is good or bad, what is proper or sloppy, what is noble or craven? It depends entirely on the context, and the values of a given society, culture or group. What each means does vary from individual to individual, and so the idea of judging someone else becomes problematic because one can not possibly know what paradigm they are coming from, right? Well no actually. Most of us live in nation states, with laws which provide a basic guide for acceptable behaviour. I do differentiate between law and ethics/morality, because law is a bare bones approach to develop a standard, while ethics/morality are often exemplary models for behaviour; doubly so because I do ascribe to the idea of virtue ethics. Virtue ethics are an interesting thing, because they differ from the more common deontological ethics, that being ethics as adherence to rules (often held to be universal). One is virtuous because they embody certain virtues, rather than following rules; one is focused on the individual, the other on what everyone should be doing. The great irony is that many people would find their conception of ethics are deontological, yet the same folks often do not believe in "judging others."
Why would people who believe that good behaviour is based on adherence to laws or rules, find judging others a problem? Doesn't the fact that there are guidelines make judging easier? It does, but you will note that I said many people would conceive deontological ethics as ethical system they follow, but this is more to do with how they conceive what constitutes ethics, as opposed to what they believe is actual ethical behaviour. This in itself stems from a hybrid holdover of a predominantly Christian world view. YHWH established a set of laws for humanity, and humanity utterly failed to live up to those standards. YHWH had to send his son to absolve people of this fact, people who accept this sacrifice are absolved of their "sins", people are then free to try their best again, but understand they will never be good enough on their own. This entire belief is alien, and belittling to me, but I'm not a Christian.
One would think that in a religion where there are innumerable laws and commandments from their deity, that they'd be willing to jump at the chance to judge others. In reality, this is actually often how it plays out, though again we come to the problem of standards; I consider myself an ethical person, but from the perspective of a Christian, I would be wholly unethical. The verse most often quoted is Matthew 7:1-6, itself depending on the interpretation of the Christian. More liberal Christians (again ymmv) would claim it is a condemnation of judging altogether, conservative Christians on the other hand, would claim it is a statement about avoiding hypocrisy. In this case, I tend to agree with the later, in its context it speaks about ensuring you are not condemning something you yourself have done. Likewise, John 8:7, states that he who is without sin, may cast the first stone, a slightly better example of what I am getting at. Since everyone is guilty of sin, humans (alone) are not able to judge others because they themselves are naturally awful. If we extricate the religious aspects, we find a common belief when it comes to ethics in the modern West, nobody's perfect.
Since nobody is perfect, how can people then turn around and judge others? "What gives you the right to judge me, you're no better than me!" I would make the argument, not being beholden to holdovers from a religious perspective I have never accepted, that this is a sentiment which accompanies someones actions who refuses to admit their mistake or take responsibility. If I have never stolen from someone, based on general cultural standards, I am in fact better (that is in terms of ethical behaviour) than a thief. If this was not the case, why is thief a pejorative and not neutral or an honorific term? If I am able to live by the ethical standards I believe in, this by default makes me an ethical person. It also means that I am more ethical than someone who has ethical standards, but does not adhere to them, regardless of my belief in a plural of ethical standards and situational ethics. Thus it provides a reasonable basis for judging others.
Why should I be held to be condescending if I live an ethical life, by someone who claims to believe in ethics, but not live by them? I accept that people fail; I have failed at any number of things. When someone has pointed out that I have failed, I do not consider them condescending for pointing out my failings, they are simply being honest. While "brutally honest" is often a euphemism for "I'm a dick"; I appreciate a tactless, but honest opinion over sugar coated platitudes. Don't get me wrong, tact is useful and it is far better to be eloquent than brash, but even this can be interpreted as being a dick.