For a text or RTF field, show the current character count in the field, next to the description, as the user is typing. This could be turned on/off in the CMS Configuration panel.
Second, allow the Schema definition to have a max chracter count per field, and the ability to warn the user when going over, and possibly not allow them to save it. The warning would be the most important.
Reasoning: Some headlines / sub-headlines break when too many characters are inserted, and we would like to wanr users that it could break. However, we would still allow them to overwrite the warning and go ahead, because in some situations they might want to do it, but at least they've been informed that it might not go well....
Also, for translations, we would want the ability to still save all the content in the field, but if an editor was to open the Component later for editing, they might see a warning that there is too much text in the field.
This option would be good to turn on/off via the CMS Configuration, because while some clients might like it or need it, during certain times of content entry / migration, they might want to disable all warnings for a short time.
Finally, the default would be off.