There are two main reasons I would like a "kick" function in parties (namely for missions). I think having either a majority or unanimous (except for the one being kicked, of course) vote required to kick someone would be reasonable. Another way to do it is to give the party leader the power to kick at will, although I think the voting would work better.
The first, and obvious reason is to get rid of assholes and griefers, or just noobs who are detrimental to the party either tactically or socially.
The second, much more important reason is that many players abuse the lack of a kicking function by joining parties for missions and then AFK-ing. Doing so, they are able to both level up easily (this is worse than botting, in my opinion, since you are leeching off of the work of others) and progress in the game more easily (if the party you're leeching off of does win, you get a free ticket to the next town). This also hurts the legitimate party members, since they get one less member to work with. I have seen this very commonly, so please, Arenanet, do something about this.
The need of a vote gets rid of possible abuse from those with the power to kick. For example, your group could get far in a mission, then kick everyone, and instantly grab all of the now unclaimed items. Also, if voting is implemented, please make the window very easy to see, such as a dialogue window popping up in the middle of your screen (and possibly have a field for the one who starts the vote to state the reason for the kicking). If the vote is difficult to see, then it's possible that newer players may not know how to kick people, and the party wouldn't be able to obtain the amounts of votes they need.
M

. But alas, the reason we need a Kick function in the first place is because of the lack thereof. I'm afraid this mechanism would lead to an entirely different form of griefing: kick-vote spamming.