I can understand the following things....
Odo sometimes having a post in Ops - He was the station's chief of security after all, a pretty important position.
Kira being on the bridge of the Defiant and even commanding it when Worf wasn't available - She's the first officer of the station, so seniority would be in effect. Also, if she's who Sisko chooses as his First Officer for the Defiant, that means she is the First Officer.
Ezri being a Defiant bridge officer - She does have 300 years of experience. She might be a counselor, but that's not necessarily the only thing she's qualified for now that she's a joined Trill.
Garak on the Defiant's bridge - This one stretches it, but he does have valuable skills that can be useful for almost any mission. Again, if Sisko wants him there, I can buy it.
Nog on the Defiant's bridge - Again, I can buy his presence by the fact that Sisko chose him to be there. However, it would have been better if they waited until he was made an actual ensign. The idea of a cadet having that important of a role is a little hard to accept.
What I can't understand are the following things....
Wesley Crusher - Why was this kid given such an important role on the freaking flagship of the Federation? As stated above, it would have been better if he was assigned to the Engineering Department. That is, supposedly, where his strengths were supposed to be. The Traveler said he was "a Mozart of Engineering."

So, why was he flying the ship?
Neelix - Aside for his sugary-insulin shot inducing-sweet view of life, what useful skills did he bring to the table? Almost all of Janeway's other Senior Staff members had some valuable set of skills to contribute. Kim was good at operations. Paris was a good pilot. The Doctor was, well, a doctor. Torres was a great engineer. Tuvok was an excellent Security Officer. Seven had a towering intellect, and was great at Astrometics. Neelix was - a cook.
Chakotay - Again, what did he bring to the table? He wasn't a particularly good leader. He often simply deferred to whatever Janeway decided. There are some exceptions, but there're few and far between. If anything, he was simply a yes-man for Janeway; more so than Nog was for Sisko.