As you already know there is no really a module Meta tags for Drupal 7 the way you knew it for Drupal 6.
Recently I came to a problem when updating my content.
I was trying to fill up the field description with phrases more related to each page content.
Then I've got the message:
Webserver encountered un error. Try later.
Well that makes things even harder, but I've realized that it was because the meta field as stored in the database has only 255 characters, so if you write too many words or character you won't be able to update.
Just keep this in mind: your meta fields description cannot have more than 255 characters.
This is kind of funny