Plugins are great. They are undoubtedly one of the best things about WordPress. But like all great things, they have quirks and we should be aware of them.
Photo by Samuel M. Livingston
1. Plugins are WordPress-specific.
I know a lot of you use Blogger (there's no shame in that!), but unfortunately, plugins aren't available on Blogger.
(I don't mention this to rub it in, I mention it because I get a lot of questions from new bloggers for whom this is unclear. You can read more about WordPress and Blogger and whether or not to switch here.)
2. Use plugins for function not fun.
Spend any time in the Plugin Directory and you'll find plugins for just about everything imaginable.
But unless it improves the usability of your blog (like tells a reader who left a comment that someone has replied), or makes blogging a lot more efficient for you (like helps you keep track of what you'll post when), I suggest you pass.
Stick to only the most essential plugins that offer the highest benefit.
3. Just because someone you know uses a particular plugin doesn't mean you should too.
This is closely tied to #2, but we've all done it. A big blogger professes their love for a particular plugin and we all race to install it (maybe thinking it'll help us be big too??).
Go easy on the plugins (especially the paid-for ones). Only install them as you need them.
For example, I mentioned two plugins above. I use one (comment notification) but not the other (editorial calendar). I've seen the editorial calendar at work and it is very cool indeed, but I've found it doesn't make a huge difference in my efficiency.
(Also regarding blog success, plugins are handy, but generally they aren't miraculous. Excellent content still wins here.)
4. Plugins tend to slow your site down.
Plugins draw on your site's resources, so if you can avoid a plugin, do.
The faster your site loads, the better.
5. If you're not using a plugin, why not delete it?
An installed plugin can exist in two different forms on your blog: activated or deactivated. A plugin that's activated means it's functioning (turned "on"). A plugin that's deactivated means it's there but not functioning (turned "off").
If you notice a plugin which has been deactivated for a while, why not keep things neat and delete it entirely? (You'll see the delete option if you hover over the plugin name under Dashboard –> Plugins –> Installed Plugins.) If you're one of those "but what if I need it later!??!" types (you know who you are ) write the name of the plugin down on a "plugins I may want to use someday" list.
6. Plugins can be buggy.
Plugins are written by all kinds of people with varying levels of expertise. Also, things online change constantly. For those reasons, plugins sometimes wig out…or cause other things on your site to wig out.
So, plugins should be one of the first things you suspect if your site starts doing funny things.
If you do indeed notice site problems, first make sure all your plugins are updated. If they are, deactivate them one by one (starting with your most recently installed one). If your site behaves properly again, it's likely you've isolated the culprit.
At that point you can delete it, try another one like it (search the Plugin Directory using similar keywords), or, try contacting the plugin author for help (this may or may not be fruitful depending on the plugin; paid-for plugins should offer some type of support). If those options don't help and you can't live without it, you could hire someone to write a new one for you.
Further reading: Here are all my posts about plugins.