Game systems, a collection of bits and bobs that can be used to play multiple games, tend to fall under the umbrella of abstract pretty easily. After all, if the pieces can be used for a bunch of different games, they can't be too unitasker. Yes, I just stole a concept from Alton Brown (he probably stole it from someone else anyway) and completely misused it.
Of course, the most important game system out there is the deck of cards. (I don't think anyone can argue that point and come out sounding rational) This would make poker and rummy and bridge all abstracts. Which could be argued but I think that makes the term abstract too vague and universal.
Random elements, hidden information and bluffing are all iffy categories when it comes to defining abstracts and card games tend to have all three. Then again, everyone says Qwirkle and Ingenious are abstracts and they have all three too.
Kind of makes me wonder if we need to break abstracts down into more categories, the way I've seen Euro games and German family games broken down. Of course, an important factor in defining a category of games is who the intended audience is and what they want out of the game. Would games-you-can-lose-money-playing be a category? Then poker and chess belong together :D
But when you look at game systems like dominoes or checkers or Looney Pyramids or Piecepack, they tend towards abstract games. But since the abstract ideal is a minute to learn and a lifetime to master. If one game system gives you a lot of games that are easy to teach but still interesting to play, that's a good thing.