So if I post several articles, and being really helpful, but people post thanks in the threads, along with feedback. That way I won't get any thanks, but I've still be really helpful that month. So the "thanks" -system can't really be used for such a feature.

Also, people might spam -thanks a special user's posts because they might be friends or something, and that way a really not- helpful member gets nominated.

And as for the problem with nominating the same user, just have it that you can't be nominated to x MOTM in a row, where x is a number of your liking.