Add info for the field and restrict to 160 characters

- Could you rename the description of the field to Meta description to avoid confusion in the default setup

Goto /admin/structure/types/manage/NODE TYPE/fields/meta_keywords
Here you can define any label yo want for your users to see but the machine names stays the same.

- Could you enclose default general guidelines for writing good meta descriptions; like: "A meta description should include as many keywords as possible, but still be a decent, readable text. The description should be no more than 160 characters long.

Perhaps have a read of what google advises:
http://www.google.com/support/webmasters/bin/answer.py?answer=79812
http://www.google.com/support/webmasters/bin/answer.py?answer=35624

- Could you restrict the field to 160 characters?

You should be able to do this at the same location you set up the field, however if you already have content in there then it is not possible to edit the maximum length. I tried looking on my site and observed that it was not an option as it expressed there was already data for this field type. Not certain if could change if just enabled that field.

- Maybe put in a counter, so one knows how many characters remains in the description.
This sounds more as a drupal core feature request. As the module is using the Field API, it depends on Drupal to manage the fields which is core. Personally I like the ID. Perhaps if you re-theme your content creation pages you can add this as a .js feature.

- Could you restrict the field to 160 characters?

That is discussed in an issue (with a possible patch):

http://drupal.org/node/1272584

Hope that helps!

Please see #1272584: Limit maximum length of field values