Positives of a common world language like English would include better communication, more productive world actions with regard to the economy, peace, human rights and etc. A common language would establish uniformity via communication.
Negatives would include a loss of individual characteristics related to countries, cultures and selves.
In today's world communication is very important. Some people feel there is a need for a common language in order to help with the communication in the business world especially. For a lot of people English seems to be the common language.
Whenever Napoleon Bonaparte conquered a country, two of the first things he had done was control of the newspapers of the country and the establishment of French as the official language. In this way, the French controlled the culture of the subjugated country. After all, the loss of a native tongue is a loss of part of a culture. For, there is a way of thinking that one acquires with each language that he/she speaks.
On the other hand, some people may feel that when millions of people can speak English, there is some unification that occurs among these people that may lead to amicable relations. Since there is no greater divide among peoples than that of language, breaking down the communication barrier may well be the first step to better understanding among nations.
The spread of English can look negative to those whose languages are being pushed aside in favor of English. This can happen a great deal in the developing world
For example, my relatives in the Philippines rarely speak a full sentence in Tagalog. They are always mixing English words and phrases into their speech. This can be seen as a bad thing because those English words can take the place of Tagalog words and that can (some say) impoverish the Tagalog language.