A similar question on why Worf 'acts' so Klingon here, but most TNG fans will note that Worf doesn't use contractions - it's quite noticeable actually - "Captain - I do not recommend that" for example, a bit like Data. I was just wondering why Worf doesn't use contractions, as it doesn't seem a 'Klingon' thing to not use them. Is there a reason for this ever mentioned anywhere?
In some circles, the use of contractions is considered too informal and dare I say, sloppy English. Worf, being a bit of a stuffed shirt would gravitate to the more 'formal' English speech patterns. Also, the more formal sounds a bit more forceful and aggressive. Note the proclamation, "I will stop you!" as opposed to the more plaintive, "I'll stop you!"
This manner of speech is more inline with his native personality and rigidly aggressive disposition.
Unlike @Morgan's answer, I think this can be merely a "sign of foreignness", not necessarily personality (Spock, Teal'c, Worf); although it is frequently a combination of the two.
This happens to be a well-known trope in SciFi, and its originator is in the same Star Trek canon: "Spock Speak" (warning: TVTropes link). Not unique to SciFi either: here's an analysis of a character on NCIS.
It is a well known quirk of some ESL (English as a Second Language) speakers - they do not use contractions, and in general use very formal language (it is harder to master spoken, colloquial, idiomatic language than a formal grammatically fixed subset of it. [Citation: personal experience]).
Worf is a foreigner twice: