Most IT companies use job descriptions listing technical skills that a candidate should be proficient in. With these lists taking the bulk of the description, little or no attention is paid to personality traits. At best, they mention something along the lines of "listening and communication skills" or "outstanding skills in working under duress and the ability to influence across all levels of the organization". I want to try to put those fuzzy requirements into simpler terms, and this is unrelated to formal selection criteria. It's hardly possible to formalize the very first personal impression a candidate makes on the interviewers, although this impression does matter a lot. This is about simple things that we feel on the gut level. It's whether you like someone, or not; whether you want to see this person in the office every day, or not; and sometimes we are not even able to put in words: what is it exactly that makes us feel aversion to someone, or to like someone.
I've written on that previously: teams are happy when they do the work they like in the company of people they like. So, which people do we like? It has little to do with technical expertise.
We like sunny persons who are sincere in their manifestations and have nothing to hide in their attitude to fellow workers. This personality trait is called integrity. As opposed to that, there are people who appear to have several versions of their self; and they turn them on depending on the social situation they find themselves in. The switcher between those versions is often based on some personal biases or fears, and this internal dividedness is somehow felt by others.
We also like people who are problem-less. I doubt that anyone wants to spend much time with a person who is always ready to grumble about something. Most of the conversations such people casually start are looping around complaints on how life is hard, or how rain is too wet, or the sun is too hot, or how their meal tastes like crap today, etc. Someone with such an outlook on life is more likely to approach work in the same way, and even an easiest move might be a problem for such people. Professional challenges need to be addressed matter-of-factly, with "just do it" attitude. The habit of grumbling and complaining makes life and work much harder. Not to mention that it's a huge energy waste.
Then, we like people who are smooth. The quality of smoothness is the one that helps most in keeping the productive flow of co-workers. If we have to deal with folks, who are not taking valid arguments and obstinately stand their ground , this is a tough challenge. One stubborn person might throw some sensitive types off track, as those sensitives would choose to shut up, only to spare themselves the trouble of confronting stubborn attitudes.
Sharing a company of people that we like plays up not only to little idiosyncrasies. IT companies pay much attention to technical infrastructure: computers, servers, all things hardware and software. Co-workers form a human infrastructure, the peopleware, that is as essential to the work outcome as the technical one, if not even more. The goal of any infrastructure is to facilitate productive flow. The less bumps in routine interactions, the more energy can be put into creative work, the stronger the flow. That's one of the reasons royal courts and other venues hire masters of ceremonies, whose job is to keep the flow of an event or a show uninterrupted. We don't have masters of ceremonies, but cherishing each other's flow and keeping the company of people we like is entirely in our powers.