Dragging files from the list at the bottom of the commit dialog to the log message edit box will insert the paths as plain text into that edit box.
Committing a folder does on any modified file in the commit dialog will launch the external diff tool to show your changes.
The context menu will give you more options, as shown in the screenshot.
If you want to avoid committing from the top level folder (with a lengthy folder crawl to check for changes) you can open the commit dialog for one folder and drag in items from the other windows to include within the same atomic commit.
You can drag unversioned files which reside within a working copy into the commit dialog, and they will be SVN added automatically.
For directories you can use on any column header you will see a context menu allowing you to select which columns are displayed.
You can also change column width by using the drag handle which appears when you move the mouse over a column boundary.
The message can be as long or as brief as you like; many projects have guidelines for what should be included, the language to use, and sometimes even a strict format.
You can apply simple formatting to your log messages using a convention similar to that used within emails.
The commit dialog will show you every changed file, including added, deleted and unversioned files.
If you don't want a changed file to be committed, just uncheck that file.
If you want to include an unversioned file, just check that file to add it to the commit.