The stamp of approval is usually an (abstract) acknowledgement that something is acceptable. 

On approval is a phrase associated with goods bought by mail-order (or distance-selling).  The customer has a short time to decide, whether the goods are suitable.  A similar idea is on probation, which relates to the behaviour of a person, either a new employee or an offender, released into the community.

Approvals are bags of used postage stamps sent to collectors (philatelists), who choose some to buy and return the rest.

Old hat means out-dated.

To throw your hat into the ring is to express an interest in doing something.  In The Dictionary of Modern Phrase Graeme Donald gives the derivation from nineteenth century bare-knuckle fighting, where a new contender would throw his hat into the ring.

Beat the system is a phrase used to describe a way round red tape – unnecessary rules and protocol.

Lead by example is advice to those in leadership or management roles.

The one and only is a surprisingly popular way of introducing people, considering that each person is unique!

Favourite may be a noun or an adjective (and has become a verb on Twitter!)  A horse may be the favourite (expected to win) or a favourite person or my favourite things – a song from The Sound of Music.

Don’t expect too much (in case you are disappointed).

It leaves much to be desired is a way of saying it is not very good at all.

To come up to scratch is to be good enough.  The use of scratch here is the same as a scratch player/team.

A promise (or half-promise) may raise one’s expectations.

A feather in your cap is a metaphor for an achievement.

Buck the trend means to go against popular behaviour.  I cannot find a derivation in my books.  Could it be from back rather than buck and mean reverse?

A step in the right direction might be a little progress.

Backsliding may occur after someone has resolved to do better.  If they have returned to their old ways they are backsliding.  Definitely not a step in the right direction!

An evil conscience is the greatest plague is a phrase I encountered in an online community a good few years ago.  This week I discussed its possible meaning with a small group of sixth-form pupils and their philosophy teacher, who was treating them to elevenses at our weekly coffee morning.  We agreed that a good conscience is one which works well and tells its owner what is right and wrong.  Thus an evil conscience could result in an increase in evil in the world and be the greatest plague.  Have you heard this saying before?  What do you understand by it?

To take something or someone for granted implies a self-centred attitude.  Granted means given.

Permission granted is a reply to May I?

Pardon granted is forgiveness (probably for a minor offence.)

I am sorely tempted…
…is sometime said by a parent running out of patience with a child.

The writer to the Hebrews tells us that Jesus was tempted, but did not sin.  Hebrews 4:14-16

An innocent bystander may be drawn into a situation or injured by accident.  Note the word innocent!  Innocent of what?  The Bible tells us that everyone has done wrong.  Romans 3:21-26