How to describe the strengths and weaknesses of white to a new mtg player?
You might not be aware but Wizards already has a page describing to a new player what the core concepts of each colour of magic are. Here's what they have to say, even more concisely than how you explain the colours:

White: The color of Justice

White spellcasters use superior tactics, efficient creatures, and the power of righteousness against their foes.

Blue: The color of Wisdom

Blue mages focus on using superior knowledge to gain control of a battle, and slowly gain the upper hand.

Black: The color of Ambition

Black sorcerers are willing to do whatever it takes to win a battle, even if it means sacrificing everything to do so.

Red: The color of Chaos

Red conjurers try to win as quickly and dramatically as possible, smashing and burning their way to a quick victory.

Green: The color of Nature

Green shamans win duels through the brute force of mother nature, summoning giant creatures to squash their enemies.

I think some of your explanations need a bit of work. For example, black isn't (always) explicitly evil, just selfish and ambitious; and red isn't just "the power of fire" (although the element of fire is associated with it) just like blue isn't "the power of water".


  White: Uses the power justice to play efficient creatures and deal with combat threats. Best at manipulating enchantments, artifact equipment, and life gain, and second best in flying. Worst at direct damage: white relies on superior tactics in combat to deal the lethal blow. Examples: Elite Vanguard, Pacifism, Lifelink.