Q2A is designed to upgrade with as few problems as possible. When you update a 0.1 version (i.e. 1.7 to 1.8) there will be database upgrades you need to run. If you have a large site you may want to look at the advanced upgrade process described here in the docs.
If you have a custom theme, there are usually minor changes required. For example when we add new features, you may need to add some CSS to handle them. If you use one of the default themes (SnowFlat, Snow, Candy, Classic) these are all taken care of.
There will also be new language strings to be translated, if you're using another language.
All of this will be documented when v1.8 is released.