>- I vs. me
>- who - whom - whose
>- their vs. there
>- plural vs. possessive (i.e. when to use an apostrophe)
>and more...

Though, some of this, I suspect, is merely a long-term trend toward a "laxity-in-declension" in English language evolution (or devolution, to some, perhaps :-) Languages do evolve, morph, or whatever you want to call it. I think the trend is to be less and less concerned over subjective/objective usage of terms like I/me, and who/whom, etc. a) It's not always worth the hassle of analyzing word structures to determine correct usage, even if you know the rules well (especially in rapid conversation, and with a lot of English 2nd language people around now), and b) it can make one seem, if always "precisely proper," appear somewhat condescending, in certain company. Just MO.

It all depends on which words and how they're abused, I think, whether there's been a blatant "foul" or merely a normal (though technically incorrect) usage. Regarding point (a) for an example, what is worse than using the objective when it should be subjective, compared to the opposite error? E.g.:

"Whom is going to the game tonight?" (Aagh, disaster!)

Whereas,

"Who are you going to the game with tonight?"

isn't so egregious, is it? This "error" is acceptable enough in most circles, and the usage of "whom" may even actually appear to be a bit too "high-brow" for certain audiences (spoken, not written, in this case). I think this 2nd misuse is well on the way to becoming a cultural "norm" at least in spoken language, if not written.

But other words/contractions such as their/there/they're and its/it's, I see very commonly wrongly applied, even in rather formal documents sometimes - and for these, there isn't an alibi of trend toward "declension laxity," or any other good excuses (short of 2nd language English, maybe). Those two situations in particular, when used incorrectly in writing, do annoy me a little.

Not that I'm always 100% perfect either, especially when doing chats and hasty 1-1 informal emails...there are other times and places to focus on language use more seriously, of course.

(And BTW, we might want to add in "to/too," to the list of annoying errors.)