Bugs of different severities have so far been put into the same category, but their characteristics differ significantly. Moreover, the nature of issues with the same severity, e.g., high, differs markedly between desktops and smartphones. To understand these differences, we perform an empirical study on 72 Android and desktop projects. We first define three bug severity classes: high, medium, and low. Next, we study how severity changes and quantify differences between classes in terms of bug-fixing attributes. Then, we focus on topic differences: using LDA, we extract topics associated with each severity class, and study how these topics differ across classes, platforms, and over time. Our findings include: severity and priority affect bug fixing time; medium-severity contributors are more experienced than high-severity contributors; and there have been more concurrency and cloud-related high-severity bugs on desktop since 2009, while on Android concurrency bugs are prevalent.