Read this http://sketchucation.com/forums/viewtopic.php?p=512726#p512726
DO NOT copy Plugins across from v2013 !
Get the newest versions - most major Plugins have been updated behind the scenes over the last few months to ensure they will work in v2014.

The PluginStore etc are a great source of many hundreds of RBZ archive format plugins that you can download and then automatically Install [Preferences > Extensions] into the correct Plugins folder, without you having to find it.
The SketchUcation PluginStore when run within SketchUp itself will even AutoInstall them for you, without the need to download a RBZ at all.
Trimble's ExtensionWarehouse is also a source of several exclusive Extension Plugins.

To find your Plugins folder use this in the Ruby Console:
Sketchup.find_support_file('Plugins')
On Windows it'll return something like this
C:/Users/USERNAME/AppData/Roaming/SketchUp/SketchUp 2014/SketchUp/Plugins
This new location removes all of the permissions issues PC users have had to endure when trying to install Plugins - it's 'yours' by default...

😮 NEVER put your own files into the SketchUp 'Tools' folder - it is exclusively for SketchUp's use...
Similarly the 'ShippedExtensions' folder contains just SketchUp files/folders.
Remove any of your own additions and restart SketchUp.

ALL of your Plugins/Extensions belong in your own 'Plugins' folder.
To make it easier to find you can always make a Shortcut to it somewhere convenient - like your Desktop... BUT using the automated installers means you might not need to access that folder very often...

To reiterate - don't blindly copy old Plugins from v2013, they might not work.
Get the latest versions they will almost certainly be compatible...
They will be in RBZ format and easy to auto-Install...
If you come across the odd Plugin which you desperately need but which is not yet v2014 compatible let the author/us know, so it can get sorted asap...
You need to remove the .rb file from the Plugins folder.
Add the Plugins in small batches so any startup errors are easier to track and trace...
What were the Plugins you had issues with ???
Help us to help you 😒