While English is a widely-spoken language, this does not mean that it is perfect. Like every language, there are some disadvantages that come from only speaking English. These range from the obvious (not being able to communicate with a large number of people) to the more subtle (little impetus to learn another language).
While English is the third-most widely spoken language in the world, it is positively dwarfed by the first two. English has around 300 million native speakers, while Hindi has around 400 million and Mandarin Chinese has 900 million. What's more, the languages below English on the scale easily overtake it.
While English is widely spoken, there are still more people who speak another language than there are who speak English. This creates problems for English speakers in many parts of the world, from China to India to the relatively-close Quebec, which is almost entirely French-speaking in spite of its proximity to English Canada and the U.S.A.
Impetus to Learn
Languages with relatively few native speakers, such as German, Dutch and the Scandinavian countries all have a very real reason to learn a second language (usually English). After all, when there are only four or six million native speakers of your language, it is very difficult to function in a global society without a second language.
English, though, exists in middle ground. There are so many speakers of English that it is not absolutely necessary to learn a second language, like it is with less-common languages. This creates an educational culture of monolingualism, which further fuels the problem mentioned above.
English is not going to be so widely-spoken forever. Mandarin Chinese, Hindi and Spanish-speaking populations are growing extremely quickly. What's more, there are three of them and only one English-speaking population. So, while English is extremely convenient in 2010, this situation will not last forever. Without a strong culture of people learning second languages from a young age, English-speakers risk being left out in the cold.