I concur with the first reply in most essentials; it makes no sense within the context of a forum post, a place where a modicum of effort in composing one's thoughts before composing one's writing is appropriate.
However, I must disagree that it is a thoroughgoing impropriety. In the context of an instant message, a medium wherein exchanges are conversational in nature and speed rather than clarity is of the essence, such a formulation not only makes sense but is de rigeur. This is even more applicable when such palavers are performed using awkward, slow interfaces such as cellphone keypads. Hence the proliferation of abbreviations such as "ppl", "l8r", "omfg", and so on.
However, even were phone text message brevity the original poster's intent, he fell short by failing to be short. In short, it's not "thx", it's "tx", k tx?
i dunno what internet you're from dbhankins, but "kthx" is a normal phrase, as are the other things you mentioned.
anyway, kthx carries an air of superiority. "suck my cawk kthx" or "make me a sandwich kthx" or something like that. and it's especially weird when someone says "im a n00b, help me, kthx". that's just weird; it kind of crosses class boundaries and makes it confusing.
plus, Killjoy just hates that phrase in the first place
im still laughing over just the idea of "im an idiot, and i rule over the land". such a contradiction with the n00b/k, thx thing. i mean its not even the regular "kthx". this one sounds like it has so much more cockiness. "yuh, k, thx."