Not casual?
Being a jerk about things...making well meaning newbs pissed by pointing out all of their mistakes and their horrible deck construction in mean ways.
P1 - ptour wannabee jerk: "Never play spells before combat newb *#@!"
P2 - (after P1 plays an instant discard spell after his draw): "what? I thought you said...
P1 - (interrupting rudely): "never take advice from your opponent! Ha ha, I win"
P2 - newb: "That wasn' very fun..."
P1 - "go home and cry about it!"
Fist fight ensues after someone gets called a homosexual and the store owner is pissed off at the behavior.
Your not helping by being a condescending southside of a northbound horse.
Casual?
P1 - nice guy FNM: "Um, that was a rather ineffective play, here's why..."
P2 - highschooler fantasy fan: "Hey thanks, yeah the art looked cool, I guess I should focus the deck more and not rely on luck to draw those two islands to play it"
P1 - nice guy FNM: "Unless you're <that guy I know>, probably not a good idea...hey I have puppies in my car, wanna see?..."
<ok, that may have been uncalled for
>
I suppose a lot of it is
attitude as mentioned before. And you know? Some people may like playing with the competetive Ptour wannabee. For those that don't want to play with people like him, I would just avoid him...unless he really is very good and will improve your skills by eventually tanning his hide every FNM until he quits to find another metagame where he can make highschoolers cry and quit the game making their parents happy they won't waste money on the hobby anymore...
"hey mom, I'll take up smoking instead. Yeah, it's a little more expensive and I'll steal money out of your purse at night, but in the long run but I'll be more accepted".
A good casual group takes a while to cultivate, where everyone's philosophies on "what is casual" are simpatico (love that word) and all have a good time.
I would suggest being inclusive, give people chances and avoid passive aggressive behavior, which is a killer of friendships before they even start. If something irks you, point it out. Sometimes people don't realize they're irking you with their habits. If you're not simpatico, leave. The joy of the hobby is that in most urban centers you'll find other folks to play with <gasp> socialize and share your interests! Maybe even find your wife or life partner?! (um...probably not) Try a local university or community college if a game store isn't your thing.
My causal philosophy: I can always improve and I seem to quit the game every other year, so my skills are rusty. Help me keep the rules straight and I'll try and keep the fun flowing with some food and try to be good company. Yeah, it's relaly important to enjoy the company you're with...how much time are you spending with this? Afterall, you'll die eventually, so don't be miserable during the time you choose to spend playing a somewhat expensive hobby.