Hello @crypto.piotr there's a contest called elevator pitch in which steemians represent the idea of steem in less/not more than one minute. Given the facts you're searching for legit and yet simple to understand explanations it might serve you well.

On the other hand, explaining delegation shouldn't be that hard.

It's like lending someone "money" without a risk of getting "robbed" or being theft victim. If ones notices that his/her stake is being misused, he can easily withdraw his delegation and in matter of 5 days he's stake will be return.

Imagine investors being able to lend money without being concerned about their stake. Furthermore, it's easy and very fast process where you can actually lend huge amounts of money without a need for third person and once you decided to do so, within 3 seconds the transaction will be verified.

Powering up should be looked at like a security measures in case your account get's hacked, because witnesses and programmers might be able to help recover it before it's to late. On the other hand, powering up literally means one has the faith in the project and is looking at it from long run perspective.
It actually means a lot in terms of sentiment and market pressure.

Upvotes - that should be easy, part of the inflation is redirected to those who're contributing towards overall economy, thus raising the awareness... Facebook wouldn't be so god damn big if there wasn't for people bringing the value...

I tried to keep it simple, hope it'll help... Cheers and good luck!!