Make fields required when using Guest Entries
If the fields are marked as required in your field layout, then they should fail validation even on the front-end, so it sounds like a bug to me.  Can you send a database backup as well as the template code you're using for the form over to support@buildwithcraft.com?

Update

The issue was that if your entries were set to be disabled by default, then Craft won't run any validation on them.  Just made a change so that if you use the latest Guest Entries, you'll have an additional plugin setting to enable/disable validation on a per section basis, regardless of the default status of the entry.