This answer is purely speculation, based on anecdotal evidence.
- Usually, your Deal of the Day is not shared among the rest of the community. This is to avoid attempting to sell items and perks to you that you already own.
- Deals can repeat. Personally, when I've seen a deal repeat, it was the same discount that it had previously been, though this may have been coincidental.
- Considering the variety of items that I have seen for sale, I suspect that all items can be put on sale, it's just a matter of waiting for it to appear.
- Deals for classes that you have not purchased will not be displayed, as you have to buy the class beforehand. Instead, you will see deals to buy classes that you have not purchased.
Also, I've seen no evidence that your playing habits affect your deals. For example, if you play a certain class more than others, it will not increase the frequency that you see deals for that class, or vice versa.