There is a funny idea out there that intelligence is a heirarchy: There are more intelligent beings and less intelligent beings. This idea is just a harmless idea on it's own. But, sometimes it's combined with the idea that the "more intelligent" beings ought to dictate to the "less intelligent" beings how to live their lives. Before we accept this notion, it's worth stepping back a minute to see what sort of ground we're standing on with our notion of "more intelligent."
What do we mean by intelligence? We could define it in such a way that certain beings are considered more intelligent -- say, with an IQ or SAT test. However, when we ask somebody to comment on the efficacy of one of these tests, they'll usually admit that what they're measuring generally comes down to how good a person is at taking the test.
That could be useful. For example, it's been shown to correlate at least a little with academic performance (possibly because said performance is typically also measured by testing).
Going a little deeper, what traits do the more intelligent have that the less intelligent do not? Aside from their ability to take a test better, can we find anything else?
Do the more intelligent better know how to survive? To survive and live longest? Write beautiful poems? Make scientific breakthroughs? Think rationally in an emotional situation? Create happiness for themselves or others? Can we find any measurable notion of intelligence which correlates to these things? Which of these things, if any, matters more to a good notion of intelligence?
One may say "I don't define intelligence exactly, but I know it when I see it." In other words, we give up on measurability. Another way of stating this approach is that we agree that there is no objective measurement of intelligence. For now. That's not to say that we couldn't discover one some day, just that we don't have one now.
1st consequence of intelligence-elitism: inappropriate lawmaking
The trouble starts when we allow ourselves to think that our so-called intelligence grants us some sort of moral privilege, or even moral duty, to make decisions for less "intelligent" beings. This leads quickly to a form of elitism where the freedom of the "less intelligent" is lost.
The problem with the idea that a few know what is best for all is that "best" eludes measurement just as intelligence does. Bust by what metric? GDP? Happiness (Yay Bhutan!)? Longevity? Creative expression? In order for it to make sense for a few to make decisions for everyone, we have to agree on the goals of each person. The trouble is, the goals of each person may be completely different! One person may want to live a long peaceful life in nature, alone. Another may want to make a lot of money, enjoy a rich life, and eventually give it all to charity. Still another may want to travel the world spreading the gospel. Who's right? If we want an inclusive society, aren't they all right?
Each person determines his or her own set of values. When we create a certain type of political policy that tries to help people, it may help some people reach their goals in life, while making it more difficult for others. By so doing, we are implicitly subsidising one set of values over another.
What does this tend towards over time? With enough such policies, a culture will tend towards a monoculture; freedoms dry up, diversity in character is scarce. Is this what we want?
2nd consequence of intelligence-elitism: miseducation
Intelligence elitism also plays itself out in most education systems at all levels. For many, it has been drilled into us from a young age that some are smarter than others (generally because they do better on the test or assignments), and that means they're somehow better, more valuable beings. We tend to forget that academic performance is often just a metric of how good we are at doing what we're told. Unique thinkers, who may possess tremendous potential, are not necessarily intelligent by typical educational-system metrics. While we're at these institutions studying, we may get so wrapped up in getting good grades or impressing our peers that we forget that Einstein didn't do so well in school, and that in fact by moulding our minds to the shape of the status quo we may just as well be reducing our potential.
Of course, not all educational systems are so narrow-minded, and not all teachers succumb to the tremendous social pressure to be narrow-minded. But there does seem to be a lot of it within our educational system.
3rd consequence of intelligence-elitism: mistreatment of animals
The notion that the human animal is more intelligent than the other animals is on the same shaky ground. Just as we have diverse forms of intelligence among people, the other animals have their own forms of intelligence. Animals can exhibit learning, creativity, cooperation, and other "intelligent" traits.
When certain people are deemed less intelligent by the educational system, perhaps "special needs" or "disabled," we certainly don't condemn them to slavery or death. Yet, that's just what we do with the other animals. Why draw a line between the human animal and the other animals? Isn't this line just as artificial as the one between whites and blacks in America not so long ago?
All this is to say, when somebody talks about how so-and-so is "not so bright" or "smarter than X", it might be worthwhile to be on the lookout for why they are saying it. Is it just an observation of the skill at a particular activity? Or is it being used to justify treating that person (or group) with less respect than another group?