Just say "no" to crotch-danglers.

I can't take credit for any of the images in this post,
and I don't know who made this one.
It's a pretty great comparison though!

1. Over-stimulation
When a baby is facing outward he is unable to turn away from stimulus.

According to Dr. Evelin Kirkilionis: "Infants in outward-facing orientations can’t turn away from surrounding stimuli. They can't turn inward toward a parent's body if stimuli become overwhelming. In this position the baby cannot make eye contact with his or her parent to evaluate facial expressions, social cues, and so forth to make assessment of the situation. Only choose a baby carrier that allows your child to face you ... never out. There are too many events going on around your baby. A baby has no way to exclude himself from the environment by turning his head away and towards you. Healthy sleep is difficult for a baby who is facing outward. I am not a supporter of the outward facing method of carrying a child."
From - A Baby Wants To Be Carried by Dr. Eveline Kirkilionis. Dr. Kirkilionis is a human development specialist who has researched babywearing since 1985

2. Poor support for Baby
In carriers like the Baby Bjorn the baby dangles by his crotch which pinches nerves and arteries in the legs and creates pressure points in the spine (and if the baby is male he is putting all his weight on his testicles -- ouch!). Babies should sit in a pocket of fabric made by the carrier and their knees should be inline or above their bum.

Examples of good support
(Beco, Patapum, Ergo)

Examples of bad support
(Baby Bjorn, Snugli)

3. Strain on mother
Forward-facing throws off the mother's center of gravity. The baby is supported by the mother's shoulders which pulls on her back, which is painful and tiring.

If the baby is properly supported, it is okay to have him facing outward for a short period of time. A good front-facing position is the kangaroo carry.

Baby carrier comparison chart
Article by Dr. Evelin Kirkilionis

"A healthy baby is all that matters"

This was written by my friend Elizabeth and is reposted with permission.

"This is not true. Having a healthy baby is extremely important, and having a healthy baby and a healthy mother is the most important thing. That does not mean it is the only important thing. When a mother undergoes a traumatic emergency situation in delivery, with lasting damage to her body (this can happen during vaginal or c-section delivery), that matters. To suggest that it doesn't because the mother got a healthy baby as the end result is another way of saying that the mother does not matter outside of her role as an incubator for said baby. This is a narrative that gets repeated a lot in our culture about motherhood - that once you become a mother your role as independent person either diminishes greatly in importance or altogether ceases to exist. It is a sexist and untrue narrative.
Additionally, telling a woman that her preferences about labor and bodily integrity don't matter sets up a climate ripe for postpartum depression. Mothers struggle enough with the idea of their own needs and wants being subsumed to those of their new baby without anyone telling them that they cannot grieve over a less than ideal birth experience.
I know someone whose husband was mugged and whose face was mutilated on the day of their wedding. If you wouldn't be willing to tell her that "the fact that you came out of it married is the only thing that matters", then I don't think you have any business telling a mother that "a healthy baby is the only thing that matters".
I personally know women who were so traumatized by their birth experiences that they have chosen not to have any more children, or waited many more years to have other children than they would have otherwise. Clearly it mattered a great deal to them, and I don't think its anyone else's place to tell them they are wrong."