In terms of 'Bug Severity' it really depends on what you want to do with the results; are you just wanting to prioritize the reolution? Or do you want to turn the bug count into some kind of metric?
The sverity of the bug depends on the over-all objectives of the implementation. E.g. in a regulated industry you will have identified 'critical algorithms' and it follows naturally that a bug impacting a 'critical algorithm' will be most severe. You may be automating a highly manual plant,in which case early user buy-in is critical, and so technically minor flaws that impact the UI may be considered severe!
Therefore bugs have got to be categorized in terms of the project's overall objectives...