What to look for in a good plugin:

• Is the plugin author providing good support? If there are many unanswered questions that is a sign of an abandoned plugin that probably has a host of other issues.
• Does the plugin come with many unnecessary features. These extra features will only add extra fluff that could slow your site down. It also splits up the developers attention. Try to find plugins that do exactly what you need, and don’t do a whole lot extra.
• Does the plugin have a good rating/reviews?
• Avoid plugins created by agencies whose primary business is not creating plugins. There time will be focused on other activities, and they may not give their plugins the attention they deserve.
• How long has the plugin been around for, and how popular is it? If it hasn’t been around for awhile it may still have unresolved issues, and if there are less than a few hundred users who have ever used the plugin, you really need to be concerned about the plugin’s stability.
• If you’re looking for a free plugin for a client, make sure you install the plugin first and check that it doesn’t require you to upgrade to a pro edition to work. Low rating are often a sign that this is happening.

Of course the above is the ideal, but sometimes you just have to take what you can get when your looking for some niche functionality. I’ve heard people claim that both plugins hosted in and outside of WordPress are safer/unsafe or more/less updated. In truth, there are bad plugins everywhere. WordPress does have a quality control system on their plugins, but so does ThemeForest. And any company such as iThemes, WPMU, etc. that focuses on creating plugins are going to be producing high quality plugins. Plugins from these companies are some of the best.

Jetpack is a classic example of a plugin you don’t want to install on your website. It is filled with all sorts of functionality that slows down your site, furthermore, they have in the past forced people to use antiquated “performance improvement” when using the plugin that were actually slowing down some websites.