Grammatically Questionable Habits
Overuse of "less": "Less" should be used in answer to the question "how much?", and "fewer" in answer to the question "how many?".
Overuse of "weil": Both "weil" and "denn" mean "because", but "weil" should only be used at the beginning of a dependent clause.
Consider, for example, the following sentence: Ich gehe heute nicht zur Vorlesung, weil ich bin müde (I'm not going to the lecture today, because I'm tired). The "ich bin müde" part is a legitimate sentence on its own, so "weil" should not have been used. Correct alternatives would be:
Ich gehe heute nicht zur Vorlesung, weil ich müde bin.
Ich gehe heute nicht zur Vorlesung; denn ich bin müde.
Neglect of "whom": Since "whom" is a such a lovely word in itself, I find it a pity that it is fading into obscurity. It isn't even particularly difficult to judge when "whom" should be used. One can simply consider the analogous "I" vs. "me" problem.
I ate the gingerbread. (Who ate the gingerbread?)
Her Majesty permitted me to enter. (Whom did she permit entrance?)
It concerns me. (Whom does it concern?)
The King and me: Some people, in their eagerness to avoid the "Bob and me went to the zoo" mistake, go too far. What results is the "Fido bit Bob and I" abomination. The best policy is to consider what happens when "Bob" is left out. "Me went to the zoo", for example, is clearly wrong, as is "Fido bit I" (unless my name is Yoda and I did the biting, perhaps).
Did you yet?: If you're going to use words like "yet" or "already", the appropriate verb is "have". Some examples:
Did you let the dog out yet? (questionable)
Did you let the dog out this morning? (fine)
Have you let the dog out already? (fine)
"They" as a singular pronoun: As we all know, this phenomenon did not arise out of ignorance, but from a wish for greater gender equality in the English language. However, even if the noble motive serves as an excuse in most cases (which I bezweifel), it certainly doesn't excuse using "they" when the gender of the person in question is known.
Would of, could of, should of: Nooooo!