Images are built and added to Glance as generic reusable images. This means that there isn't any data included to launch the image that is built into it. To provide the image with configurations to allow login and customization, the images should include a service called cloud-init. Cloud-init calls back into OpenStack to get SSH pub keys and post-boot configuration commands. There is a predetermined URL that cloud-init calls into: http://169.254.169.254. If you are getting an access-denied error when you try and SSH to your floating IP address, it is probably because cloud-init is failing to get the SSH pub key for your authorized keys file, you are using the wrong username, or you are using a prepackaged image that you have downloaded with a username other than root.
To troubleshoot the metadata service, make sure that you have an image that you can connect to the console. CirrOS is a good option for debugging things like this; just remember not to use...