The Austro-Hungarian Empire was huge multi-national empire. Some of the mayor nationalities represented, other then the Austrians and Hungarians, were Czechs and Pols. The empire was a dual-Monarchy, because of the Austrians defeat under the Austro-Prussian war. The Austrian empire was weak and in need of a new government. The Hungarians grasped their opportunity to demand more power and a dual-Monarchy, which made Franz Joseph I both the emperor of Austria and the king of Hungary. Some of the other large groups of people (i.e. the Czechs and poles) in the empire felt it unfair that the Hungarians should get power and the right to rule over their part of the empire when they themselves had no influence. Therefor one could say that it is due to great ethnic diversity and dissatisfaction within these ethnic groups that the Austro-Hungarian Empire was weak and unstable.
The smaller ethnic groups were under represented in government, were less lightly to get good jobs and jobs as officials and were forced to learn German as well as their own languages. In the Austrian part of the empire the government decided that everyone was to learn not only German but also Czech. This seemed fair enough to the Czech but to the Germans this was terribly unfair. The minorities in the Empire were all forced to learn German anyway but the Germans were not used to having to learn any other language than their own. Even so there were many of smaller groups that still felt ignored and were unhappy.
On the Hungarian part of the Empire everyone was forced to learn/speak Magyar (the Hungarian language), this was very strict and in schools teachers could loose their job if their pupils did not speak Magyar. This meant that all minorities being suppressed and felt like they were being treated unfairly. Therefor there were several nationalistic movements demanding political and cultural equality.