What folders can be safely excluded from Time Machine backup?
The simple answer is to exclude any files that you create or download that you won't need for a restore (this includes files that you're able to re-download or restore/rebuild from another source if necessary).

Time Machine does not actually backup everything. On the contrary, by default it is configured intelligently to exclude folders like ~/Library/Caches, ~/Library/Logs and many others that are temporary in nature, can be rebuilt from available data and are not required during a system restore. Adding these folders to your exclusion list is not necessary and is redundant.

The default list of files and folders that Time Machine excludes from its backup is listed in /System/Library/CoreServices/backupd.bundle/Contents/Resources/StdExclusions.plist. You can verify that the caches and logs folders are in this exclusion list.

For more information, see:


What should I exclude, and what should I not exclude?.
How can I find exactly what files are included in my last Time Machine backup?
On OS X, what files are excluded by rule from a Time Machine backup?


Personally, I exclude two kinds of folders:


Folders that contain temporarily used files that I can get from another source if I need them.
Folders containing large files that may change frequently. For example, large media files that I may add and delete often are excluded from Time Machine because they would fill up the backup disk very quickly. Those are, however, covered in a full disk clone (that I manage with a limited backup of older files).