Plugins are a bit like medication. Take one pill and the results are fairly predictable. Take two different drugs and all should be well, but once several or even dozens get into the mix, the results can be deadly.
WordPress does a good job of screening those plugins which get listed in the docs, but no one could guess how they will all interact with one another. Most blogs use many of these handy add-ons and employ a process of trial and error to get everything running smoothly.
Themes are another complicating factor. It is impossible for developers to test their product with every available theme. WordPress.org currently lists 51 pages of themes and 424 pages of plugins. There are of course many other third party add-ons also available. The possible combinations are astronomical.
When things go wrong the only recourse is to deactivate one plugin at a time until the problem clears. This still leaves unanswered the question of where the conflict lies. Is there a flaw in the one deactivated or does it just not function in combination with one or more others? The solution is to decide what is really needed and what can be done without.
What is the safe course to follow? Limit plugins to the minimum needed to get the results looked for. Don’t keep installing and then deactivating as this doesn’t always remove all components, and those left behind can cause trouble down the line. Backup and do it often! If something goes horribly awry at least it won’t be necessary to start from scratch. Be suspicious of products not listed by WordPress, they may be useful or prove disastrous. Don’t try to do it all at once. avoid installing several plugins at a time. Work with changes for a while before the next move. Taking one step at a time can save time and aggravation.
WordPress is clearly the premier blogging platform. It is almost limitlessly expandable, but the price is a steep learning curve. However meeting this challenge is one of the joys of starting up a new blog.