In my humble opinion, the major differences between L1 acquisition and L2 learning are:
1. Most people acquire their mother tongues when they are children. Prior to puberty, anyway. At this time of life, there are several physiological factors that make acquiring any knowledge easier, but especially one's first language. Generally speaking, children's brains are like sponges and information is soaked up much more readily than after they become adults. Also, the bones of pre-pubescent children have not solidified in their final or adult shapes. The bones of the jaw are some of these that are still not completely formed, making it physically easier for a child to make new or unusial sounds over an adult.
2. When one acquires L1, one is speaking the lingua franca, or the same language that everyone else is speaking. Thus, it is normal. There is nothing unusual about it, there is nothing that makes it in any way a burden. Practicing the language is as easy as turning to anyone and simply speaking. Learning new words is as easy as pointing and asking.
This is unlike many people's experiences acquiring an L2. Often, students acquire an L2 in a setting where most people do not speak that language, making practice difficult. The necessity of learning the language is often questioned and the more necessary it is to learn a new language, often the more burdensome it feels. New vocabulary is sometimes difficult to acquire; one must make sure that whoever is asked is one who has the correct knowledge. Often, if a word is obscure, nobody seems to know.
3. When a child mispronounces a word, it is normal--even cute. Not so for an adult. An adult is given less grace when he or she mispronounces a word and is seldom considered cute for doing so. If the child continues to mispronounce a word, his or her peers will make fun of the child. The same is true to some degree with adults, as well, however, while this may be a traumatic way to learn, it is less traumatic to be made fun of as a child than as an adult.