THIS IS A REALLY GOOD QUESTION!
But let's start from the meaning of the strange word wiiki!
The word wiki comes from the Hawaiian language and it means "quick". Nowadays it refers to a database or a website developed collaboratively by a COMMUNITY of users. This word was coined by the US programmer Ward Cunningham, who stated "I chose wiki-wiki as an alliterative substitute for 'quick' and thereby avoided naming this stuff quick-web.". Some also think it could also be the acronym of "What I know is..", which describes the function of a wiki: sharing knowledge and acquiring new information.

WHAT'S A WIKI?
Wiki is discussion medium and tool for collaboration, anyone can just come along and put whatever they like on any page( comments, impressions)
Wiki uses open source software to collaboratively write and re-write what others have published.
It is a sort of half finished work because anyone can get around and get involved on the discussion or on the topic.
A successful example of a wiki is Wikipedia, which was started in 2001. It's a free and multilingual encyclopedia. It is a collective project where anyone can edit or post entries.

Obviously you can't put whatever you want in wiki, there is an "etiquette" to follow, the so-called "wikietiquette" that clearly express what you can and cannot do in a wiki. A wiki can be considered a community and expects correct social behaviour.First of all you have to be polite, you shouldn't offend people or send spam, you always have to be objective but never bitting otherwise your post will be not sent. The most important thing is that you have always to be CIVIL.

As far as the style is concerned, the language in wikis is quite formal because it should be understood by anyone who comes to visit the webpage ( even by non- native speakers). Local slangs and acronyms are( or should be) reduced to a minimum. However, surfing in some wikis I noticed that in forums, which are shown in almost every wiki, the language is more informal, and abbreviations or local expressions are present in a large number.


When I was exploring wikis I tried to understand why they decided to buid up a wiki, and I found out that is main purpose is freedom, but also the opportunity of improving webpages by adding comments and knowledges.
At the beginning I thought that it was more or less a sort of forum, but then I realized that a wiki is more, it collects all the important arguments, and then you can easily search for them on net.

I think that like weblogs or podcasts,wikis have started to become popular online as people find that it's a new stimulting way to create and to express themselves, but also to know what other people's knowledge is.