1. Bad posture/looking like a silly.
2. Stalled progress.
3. Muscle and joint injuries.
If any of these things sounds good to you, then be my guest and keep training without considering superior/inferior, anterior/posterior and medial/lateral muscular balance. I know that being a weak hunchback with knotted muscles and arthritis sounds appealing to me at times, so I wouldn't blame you.
My body goals for 2013.
Now, this is an area where programming can become a bit more of an art than a science. There's definitely science involved in knowing your anatomy, but applying your knowledge of anatomy in a way that's effective, efficient and pushes you towards your goals is very much an artform.
A passable understanding of anatomy is required here. Pro tip: if your exercise routine results in you getting sore in/growing muscles that you didn't even know existed, you probably shouldn't be writing any programs, since your understanding of anatomy probably isn't up to scratch just yet. That's not an attack, by the way, and I thoroughly believe that if you've read this far, then you have the ability to learn, and therefore down the track when you're more familiar with the human body you may be able to put together very good programs indeed.
I can't possibly go into details on all the ways in which people go wrong, so I'm just going to mention three generic issues that tend to arise with beginners. These issues tend not to happen so much with advanced lifters for a couple reasons. Firstly, as stated above, muscle imbalances, caused by imbalanced programs, tend to result in stalled progress (preventing someone from advancing) and injuries (preventing people from even training). Secondly, the people with the mentality required to become advanced lifters will also have the mentality required to do the research and learn about balanced exercise program on their own, or they'll be willing to defer the programming to someone they can count on to make the right choices for them (a competent trainer, or a coach in whatever strength sport they'll likely be involved in by the time they've reached an advanced level). Anyway, here are the three generic issues:
1. Mirror muscle training.
Boys are more notorious for this than girls, but both sexes have plenty of culprits here. Your "mirror muscles" are the muscles you can easily see in a mirror. A typical mirror muscle routine will involve exercises such as bench press, push ups, overhead press, bicep curls, sit ups, crunches, 30 other ab exercises, and maybe leg press and leg extensions. If you're lucky, the people who do these routines might add in some lat pull downs or chin ups, hamstring curls and calf raises.
Unfortunately, "out of sight, out of mind" doesn't make the posterior chain any less important for your musculoskeletal health and longterm progress. Even the few back exercises that may be used are often not the right exercises to balance out the mirror muscles, and are often done with lacklustre effort and/or form that favours the mirror muscles anyway. For example, if you train in a commercial gym, you've probably seen (or have been) someone doing lat pull downs, and instead of keeping their chest up and pulling with their lats and lower traps, they curl their trunk forwards, using their pecs and abs to pull the weight down. The technique, volume, intensity, and/or frequency required to make any posterior chain exercises effective just isn't there, if the exercises are there in the first place.
Mirror muscle training tends to result in bad shoulders, back pain, and knee issues.
2. Upper body only.
This is mostly a male thing. Guys want big arms and a big chest with 6 pack abs and a V-taper, yet have no interest in training their lower body. This is often coupled with mirror muscle training, and the mentality behind it tends to be the same: "Chicks dig big arms, a big chest and shredded abs, and no one can see your legs anyway." Funnily enough, guys with this mentality seem to be in it to attract women and increase their odds/frequency of having sex. As a Christian I could get into a great big moral lecture about that, but putting morals aside, apparently these guys are lacking the foresight to consider that sex usually occurs with the pants off, in which case she most probably will see your legs. I'd like to say that this tends to result in people looking like Jonny Bravo, but most people either get into more holistic goals or quit before that ever happens. You do see the occasional guy with 30 inch arms and 15 inch thighs, though.
Don't worry, you won't actually end up looking like this.
You'll quit before you gain an ounce of muscle in the fist place.
When paired with mirror muscle training, the same shoulder and back issues are likely to occur as per mirror muscle training. If you do happen to train the upper body properly, but just omit training your legs, you probably won't hurt yourself...physically. Your pride might take a beating, though, when you wake up one day and either realise that your legs look ridiculous compared to your upper body, or that you've been training your upper body for 8 years and nothing's happened (oh wait, that sounds like skinny old me). Of course, strong legs allow you to stabilise yourself properly while lifting, and so the absence of quality leg training may limit your upper body performance, eventually putting a cap on upper body progression, but most likely if you get no results while not training your legs, it's because the same ignorance and/or unwillingness to learn about/apply leg training has kept you in the dark about/prevented you from applying other habits that would result in progress, like proper eating.
3. Lower body only.
This is one for the girls (mostly). Girls come to the gym wanting toned legs and a hot bum, and behave as if they're allergic to upper body training. Upper body exercises are either too hard/scary because "I can't lift that!!" or they're too hard/scary because "I don't want big arms and big pecs like a man!!" Or it's a combination of the two. Or it's something else entirely. Whatever the story, the same chicks that apparently dig big arms, a big chest and shredded abs on a man and won't see his legs anyway (according to his own reasoning above) tend not to want those same traits on themselves. In fact they tend to be the exact opposite. Lunges are to these girls what the bench press is to those guys, and good-girl/bad-girl machines (more professionally known as hip adductor/abductor machines) are to these girls what bicep curls are to those guys.
Guys with giant upper bodies but legs that are only fitting on Tweety bird have become a classic caricature of the dumb jock stereotype. However, the same caricatures aren't normally applied to girls with "toned" bums and thighs and no evidence of having ever trained the upper body. I suppose, at an extreme you might encounter some massive Ghetto booty caricatures, but they're seldom associated with fitness...rather they're associated with fatness and/or blackness.
Suffice it to say, toned legs, a toned bum and an upper body with little muscle mass is often considered hot when it's on a woman, so, when it comes to generalised standards of beauty, there's seldom much of an aesthetic negative to girls in the lower body only boat.
Functionally, however, this may be a different story. For starters, women still need to carry things and pick things up. You can't rely on men to do everything for you (even though doing manly things like opening jars for damsels in distress does help to validate our existence). In the gym, squats and deadlifts are some of the best builders of the bum and thighs, but if your upper body isn't strong enough to handle the weight, then you won't be able to do those exercises effectively. Even the leg press loads the weight all throughout your torso, which requires a strong trunk. So, outside of the gym, neglecting your upper body can be problematic, and inside the gym it can prevent you from getting the most out of your lower body training.