One definition of "casual" per www.dictionary.com:
Being without ceremony or formality; relaxed: a casual evening with friends.
So, what is the casual player? Does it REALLY matter exactly what the casual player plays with? Based on the definition provided above, no. One interpretation of this could be that a "casual" player is one who could be considered by most of us as someone who plays only formats that break the basic rules of Magic: The Gathering, much in the way of "15 Card Highlander" that Gizmo introduced on StarCityCCG a while back. Or, the casual player is someone that plays 'relaxed,' i.e. without stressing over winning, and simply has fun. Either way, the casual player is someone who well and truly enjoys playing Magic: The Gathering for various reasons, but plays for fun without stress.
So, what cards can be directly attributed to the casual player? One word: All, with a stronger emphasis on the uncommons and commons.
Think about it for a moment. When you play Magic: The Gathering, is it because you have to prove something or win something for various reasons? Or is it because you simply enjoy Magic: The Gathering for what it is: a game?
Personally, I feel that I'm casual because I can't keep up monetarily and time-wise with other players that play in tournaments (notice I do not specify casual and tournament players in this statement). These people are truly devoted to the game in a way I simply can't be. However, I do buy my share of cards. I do like to build a variety of decks (not combo decks, I usually simply don't have the cards). I also really don't like being so cutthroat. It makes me feel dirty if you consistently wipe someone out with a tourney deck. Creative decks are offbeat, interesting, and sometimes unpredictable for both the opponent and player. Finally, I do like the entire bit of player interaction that Magic provides.
As a casual player, I have more commons than any other type of card. Since I can't keep up monetarily with any of the environments that the DCI sponsors (Limited aside), I focus more on the commons and uncommons available to me than rares. Sure, I have my four or more of my favorite rares (I have seven Mana Crypts, for example, shelled out for a Collector's Edition Mox Emerald which I prize prodigiously, but I also just had to have 4 of the long-awaited Elvish Champion). Zadok, for example, is the 'Greater Good Fanatic.' We all have our special or favorite cards, relaxed player or not.
The casual player, more often than not, sifts through the commons and commons for the gold for their particular creation. Of course, it comes down entirely to what you like to play: Timmy, Spike, or the last one (GOD, what is that name???). Luckily, R&D have pushed out some great cards for the casual player: the new "Dual" lands, Rancor, and Blastoderm are prime examples. I really do feel that since Mercadian Masques, R&D has not only succeeded in slowing down the environment, but also have begun to truly cater to the masses of this game: the "Casual" player.
Is there anything wrong with being a casual player? Let me play devil's advocate for individuals such Gizmo (a VERY strong proponent of tournament play here at the CPA): Is there anything wrong with being a tournament player? I have to say "no." I can hear a collective gasp from you all...I don't say yes because I'm partial to tournaments, but because these guys just enjoy playing Magic too, and some of them have some serious money on the line (what better incentive to take it seriously, eh?). Sometimes, I feel there is a stigmata applied to the "tourney" player. Is that really any worse than someone calling a casual player a "scrub" for playing an Elvish Champion deck is?
To sum up, I feel that we sometimes forget what the cards of the casual player are: They are what you want them to be. They are the cards you like for the fun and enjoyment they give you, regardless of rare, uncommon, or the ever-so-maligned common. They are the cards with the "WOW!" artwork. They are the cards that continue to inspire the imagination and bring to life vivid worlds. They are the cards that you can put into a deck and play with, not because the metagame requires you to, but because you simply want to. Those cards are the cards of the casual player.