- China has the world's second-largest economy and - probably - the world's third most powerful military. Its relative influence, however, is still limited. China today is far more of a great power than it was in 1945 (indeed, in 1945 it probably didn't deserve to be labeled a great power). Moreover, China is indisputably becoming stronger.
France - France has the world's fifth largest economy and a very modern and powerful military, probably in the world's top five. On the other hand, its influence is somewhat limited outside the former French Empire. Compared with 1945, France is substantially less of a great power, having lost its empire and fallen under the American umbrella. Indeed, like most of Europe it has been in relative decline ever since 1918 and looks set to continue to decline in relative terms. This is because the Third World is slowly catching up to the First World, rather than any fault of France itself.
Russia - Russia has the smallest economy of the five, barely (or not at all) breaking into the world's top ten biggest economies. However, Russia's military is unquestionably the world's second strongest, and it dominates the region it is located in. Russia fell into steep decline after the fall of the Soviet Union, when it was on par with the United States, and has only recently begun to recover.
United Kingdom - The United Kingdom has much in common with France. Its economy is the world's sixth largest, and its military is probably in the world's top five. Nowadays, the United Kingdom's influence is more cultural than anything else; it neither dominates Europe or the former British Empire. Out of all the powers, the United Kingdom has declined the most since 1945 - losing both its empire and economic preeminence.
United States - The United States has the world's largest economy and most powerful military. It strongly influences the entire world. It is more powerful than in 1945, with the fall of its great rival the Soviet Union.
All in all the United States, Russia, and China (going in order of their great power strength) definitely ought to be in the Security Council. The case is more questionable for France and the United Kingdom. Europe is still a very powerful entity in the world and should have a permanent member in the Security Council. But having two members in the Security Council - as is currently the case - certainly overstates its status.
The trouble is that by themselves, France or the United Kingdom aren't powerful enough to have one seat. Nor is the European Union influential or coherent enough to deserve a seat. Under an ideal situation, one-third of a seat each would go to France and the United Kingdom, with the other third going to Germany. This, of course, wouldn't be feasible in the real world.
Finally, let's take a look at the countries which some propose adding as permanent members:
Brazil - Brazil has the world's seventh or eighth largest economy, which is why people propose adding it. However, Brazil has no substantial military presence to speak of. Its influence is limited to Latin America (where the United States is probably more influential). While Brazil has become relatively more powerful since 1945, it is still not in the category of great power status.
Germany - Germany probably has the strongest claim to being added to the permanent Security Council. Germany's economy is the world's 4th largest (bigger than the United Kingdom or France), but its military is still quite weak due to the restrictions imposed upon it after World War II. Germany is generally seen as Europe's first-among-equals; it is Germany, not France or the United Kingdom, which is coordinating the response to the European Union debt crisis. Germany has thus definitely become more powerful after rising from the ashes of 1945.
India - India is similar to Brazil in many respects, except weaker. It has the world's tenth or eleventh biggest economy. Like Brazil, its military is essentially nonexistent. It has very little influence even in its neighborhood. India has certainly strengthened since 1945, when it was under foreign rule. However, it definitely is not yet a great power. One could make a stronger case for adding Italy or Canada to the permanent Security Council than India (or Brazil, for that matter).
Japan - Japan is a unique case. Its economy is the world's third largest, which seems to say that Japan ought to be included in the permanent Security Council. Japan's military, however, is extraordinarily weak. Furthermore, Japan has no regional influence; it is regarded negatively by its neighbors for its crimes in World War II. Indeed, Japan has been bullied quite recently both by Russia and China over disputed islands, with Russia and China getting the better of it each time. While Japan has advanced economically since 1945, its regional influence is still lower. Before World War II, for instance, Japan occupied Korea and much of China as a colony; this would be impossible today.
Out of these four countries, probably only Germany truly ought to be in the permanent Security Council. Brazil and India are still middle powers. Japan, while economically strong, lacks the other qualifications that go along with Great Power status.
Indeed, none of these countries have been able to exert their strength in ways the Security Council Five have in the past decade. The United States invaded and occupies Iraq and Afghanistan, countries half around the world. Russia invaded Georgia. The United Kingdom and France played a major part in bombing Libya. Perhaps only Germany - and even this is fairly uncertain - can do something similar today.
The world has changed a lot since 1945, but it has also changed a lot less than many believe. The five great powers in 1945 still are, by and large, the five great powers in 2010.