@dynamicv
 
Heart+Apple -> Compassion (Diamond Tiara, Zecora) and Love (her family);  
Star+Music -> Magic/Friendship (with her friends) and Harmony (with her sister);  
Lighting+Wing -> Willpower/Speed and Hope (that one day she could fly);

@FanOfMostEverything  
Their cutie marks reflect their ability to help others find their cutie marks. Everything beyond that is just fluff.

@Scootaloo is best pony  
It’s a heart in an apple in the Crusader shield. Which is apparently a thing.

“We’ve got HEEEAAAAARTS!! As strong as horses!!!”

Is her mark an Apple or a heart? I can’t tell.

@Philweasel  
… Yeah, that works. Thanks.

Applebloom’s biggest trait has always been her willpower, her drive and ambition. Her… well, heart.
 
It sometimes steers her wrong, but in her very first appearance with Zecora she knew what was right, and wouldn’t listen to anyone who told her otherwise.

@Fwelin  
Yes, nor does Dash have anything for incredible observational abilities. But the heart just feels like filler to me, like they couldn’t think of anything better.

@FanOfMostEverything  
That just means it’s a lesser talent. Twilight doesn’t have any references to her organizational skills in her cutie mark, for example.

@FanOfMostEverything  
The Shield represents the Cutie Mark Crusade (their calling, to help young ponies get their marks)
 
The three colored bands represent the Cutie Mark Crusaders themselves, it is their hair colors.
 
Each of them has a symbol on their shield that differentiates them:
 
Sweetie Bell has a musical note in a star, representing her love of signing.
 
Scootaloo has a wing with a bolt inside, representing her love for high speed stunts and her thematic connection with Rainbow Dash, who also has a bolt.
 
Applebloom has an apple with a heart inside. Obviously the apple connects to her family for thematic reasons (plus she has some skills that are useful on the farm), and the heart is… compassion I think? Diamond Tiara was bullying Applebloom longer than the other Crusaders, she should hate her the most, but was the one who first suggested they should help her.

While it’s great to finally have a canonical cutie mark for Apple Bloom, it feels a little generic. I was hoping for something reflecting her engineering ability, either physical or chemical.