In biology it's very important to understand situations in which evolution will not occur so that you can understand when it does and why. A system that will not evolve is spoken of as being in Hardy-Weinberg equilibrium. Here are the conditions that would have to be met for that and then we'll discuss our exceptions to that.
Humans would have to have...
A huge population. We got that down pretty cold and are working on improving it. This is a double edged sword, though. It stabilizes genetic drift (the random wander of evolution), but it also increases the amount mutations that can be selected for. More on that below.
Completely random mating. Condition not met. 93% of marriages are couples who married their own race. Besides, by in large nerds marry nerds, jocks, jocks, etc.
People are all 100% equally as likely to both survive and reproduce. This is blatantly false. Just look at the country demographics. Some countries in Africa the women have on average 6 or 7 kids while some European countries are in negative population growth. Furthermore, if you've ever watched the movie Idiocracy
Lastly, there'd have to be no mutations in DNA transfer. This, also, is of course not met. Each baby comes out about 2 or 3 mutations different from her/his parents (hundreds of times more than that actually, but since there's so much "junk" DNA in us it doesn't manifest itself). O, and 90% of those mutations came from your dad. It should be noted, too, that this will only increase as couples trend toward getting married later and later.
Despite essentially none of those conditions being met it's still a controversial subject. Hence, why it's nice to be able to put forth some examples of evolution from the recent past to show that, yes, it really is happening. Lactose digestion is around 8,000 years old and yet in certain countries like Denmark and Holland the percent of people that have it almost reaches 100%. Based on gene studies, blue eyes are less than 10,000 years old. That being the case, it should be ridiculously rare unless it had some edge over other colors. You can look at the skulls of earlier humans and see that our skulls just aren't the same. Within the last 40,000 years they've gotten measurably smaller by several hundred ccs and higher vaulted to increase the frontal lobe size. Interesting to note that we exhibit much of the same changes that domesticated animals, like dogs and sheep, skulls undergo - smoother, taller, more spherical skulls.
Next, as a review there are two ways in which true evolution occurs. One is that a survival advantage is paired with a particular mutation - longer neck makes some giraffes get more food, a thicker beak makes cracking more nuts possible to eat, a variant color moth conceals itself better etc., etc. That's known as natural selection, of course. But, there's another equally important factor that must be considered in these discussion and that's having a differential reproductive advantage - some people have more babies than others. This latter factor is by necessity going to have to be the focus of our discussion since modern medicine has such an effect on the former.
First, let's consider natural selection and genetic drift. For millennia humans would die if they had bad enough vision to not be able to hunt/flee predators. That's no longer the case. For just as long humans likewise been offed if they had couldn't fight off a pathogen or deal with a genetic disease. That's not true anymore. So, what will the net effect be in our evolution? Well, it means that genetic drift will take over where natural selection dropped off. Basically, the mutations will continue to accumulate in parts of our genome that aren't being selected against. Fined tuned systems like sight, hearing, metabolism, immune system, etc. will start to break down. We can already see this in play with our sense of smell. The majority of our smell genes (51%) are mutations that have turned off our ability to smell many, many different scents that were formerly important in our ancestors. This can account for why some people like some foods and it makes others want to hurl - different mutations. Another that's made it through the cracks is our ability to synthesize Vitamin C. Prior to primates our ancestors used to be able to make this vitamin on our own. It's kind of a handy thing to be able to do, but since our diet often included items that were high in Vitamin C, like fruit, we didn't have to. Use it or lose it is the name of the game.
We've come to my favorite topic now - sexual selection. What attributes are being selected for by sexual selection? Meaning, why do some people have more babies than others? Well, it might be easiest to contrast this with what used to be the case.
Formerly intelligence was selected for. Males that were brighter gained more status thereby gaining more mates. Monogamy is killing this and the career over family focused intelligencia is reversing it.
It should be noted, however, that this trend isn't unidirectional. Meaning, it isn't just that smart people are having less babies. Severely mentally handicapped people are having less, too, (causing stabilizing selection) and thus enters rape into the discussion. For time and memorial rape has been a sadly significant reproductive strategy for humans. Couldn't get a mate? Make one. Rape selects for completely, completely different characteristics--aggression, victimization, dominance, anti-social behavior and even possibly retarded intelligence. Nowadays those that are aggressive and antisocial won't reproduce if they can't get a stable long term relationship. That's big. Don't underestimate the evolutionary power of that. I see a glimmer of hope in this. Nicer, gentler, more social people can reproduce more. Yay!
What else might be selected for? Social skills like fluency, empathy, attachment, risk taking behavior that results in teen pregnancy, decreased standards and pickiness, more attractive features, resistance to getting fat (for health and reproductive reasons) from a high carb diet (we've only eaten grain for less than 10k years), horniness and I've read that women that are shorter and stockier are more fertile than the gracile, twiggy models of the run way.
In summary, it isn't possible to summarize because it's possible everything I just said is either wrong or irrelevant because of future genetic modification. haha! That's not entirely true, actually. I think the take away summary is that while humans may get both dummer and less healthy we may also become nicer, more pro-social, more empathic beings that may, because intelligence is always communal and synergistic, end up, as a systemic whole, become more intelligent. But, who knows? 600 million years ago before chordates you never could have predicted the dinosaurs. 65 million years ago when the dinosaurs died out you never could have predicted humans. We struggle to predict the weekend's weather, 50 million years from now is impossible to predict...but so fun to think about!!!