How to launch jar the same way Jar Launcher does
The problem is that you need to specify the class paths with the cp option to java and the library paths with the D option. The Minecraft launcher under /Applications handles this if you have a default install.  If you want to run the default configuration from the commandline, just run something like the following with the memory values of your choosing:

java -Xmx2048M -Xms512M -jar \
/Applications/Minecraft.app/Contents/Resources/Java/MinecraftLauncher.jar


Here's an example of how I run Minecraft from the command-line for ad-hoc mode.  I put the following in a script, name it and run it by name:

MCHOME="$HOME/Library/Application Support/minecraft/bin"
CP="$MCHOME/minecraft.jar:$MCHOME/lwjgl.jar:$MCHOME/lwjgl_util.jar:$MCHOME/jinput.jar"

java -Xmx2048M -Xms512M -cp "$CP" \
    -Djava.library.path="$MCHOME/natives" \
    net.minecraft.client.Minecraft