I agree about loyalty and honesty, and this makes me sad.

Important bit:  
Really, Luna betraying Celestia isn’t necessarily proof she wasn’t loyal; Keep in mind, loyalty doesn’t exist in a vacuum.  
Heck, if she WAS loyal, to her sister and to the citizens, it would hurt all the more to be ignored.  
She’d eventually try to FORCE people to pay attention to her - and the Nightmare Moon aspect made it more violent than it’d be otherwise.
 
Longer rambling:  
I suspect without becoming a Nightmare, she’d do the grown-up equivalent of a temper tantrum, to which Celestia would probably react with shock (especially in fanon; We don’t know too much about the canon attitude back then, but the likely one is what fanon holds: She was too busy being a ruler, and possibly a bit lost in the admiration others had for her, so she didn’t know her sister was hurting)  
Without the Nightmare, Luna wouldn’t do anything bad enough to be banished, but would likely either try to create her own realm inside Equestria (ala ‘children of the night’, though that exact method might not be the best :P), or cause enough strife and trouble that Celestia was forced to punish her in a lesser way.  
The whole conflict, which to be clear would be at least partly Celestia’s fault, would set the whole ‘peaceful utopia’ back quite a while - not entirely sure it could ever happen before the royal sisters made up (the two will likely never fully agree, they are too different. In order for the place to be run without conflicting laws and such, one of them has to be willing to trust the other to be right. It’s unlikely to be Celestia, with her precedence and habit of acting as the primary ruler, and evidence implies it SHOULDN’T be Celestia (she did usher in nearly 1000 years of peace), so Luna has to be willing to act as a second to Celestia)  
Though on the plus-side, if it did work out and she spent the thousand years with a near-equal by her side, Celestia might have ended up acting less manipulative and bored - I don’t fully agree with the ‘Trollestia’ portrayal, but she does clearly enjoy (lightly) ‘trolling’ ponies who are too tense; The Cakes for example, and if the ‘important ponies’ are anything like real-life nobles, chances are they were at the tripple-G to build connections and alliances (NOT to have fun, though some light fun on the side might be excused), especially with the ruler, so interrupting them with chaos for the sake of making the event more enjoyable for yourself IS trolling.  
For other events, just look for justifications/examples of ‘Trollestia’

Nightmare Moon is obviously the inverted tarot card of Luna; treasonous, deceptive, and cruelly mocking. She’s the negative expression of the traits Luna embodied.

Those harmony virtues sound about right, considering the Luna episodes and the ways Celestia is portrayed.

I think they represent the elements to some degree but not enough where it matters.
 
Celestia has shown she’s kind and generous to ponies and everyone. So those two fit and god knows she’s got a lot of magic. So does Luna but lets assume she’s got more.
 
The first thing Luna did was say she was sorry to her Sister. Showing a bit of Loyalty. She’s shown amusement in being with others even though she’s unused to it. Laughter something she needs more of. Honesty, she helps the fillies in their dreams showing the honest side helping them out.
 
Then lets look at how those elements match up to Nightmare Moon. One who is brutally honest is often considered Cruel. If one’s loyalty is never returned they’re being taken advantage of. Even if they find it funny it may seem dark or cruel. [like the spiders. that was funny]

@Background Pony #1106  
Or as I said in my comment it,s because of facing the danger by laughing. Idon’t think she was like Pinkie but maybe she was not so conservative as now

I think they embody those elements quite well, and not only that they fit well with their respective celestial bodies.
 
For example the Sun is like Rarity and Fluttershy in that it gives off heat and light without playing favorites. The light it gives off reveals the beauty of the scenery but it itself is blinding when looked at which seems to say that it wants you to look at others and not itself.
 
The Moon on the other hand does the exact opposite, the darkness of the night does not allow for much else but the beauty of Moon and stars to be seen. Imagine for a second if everyone just ignored Rainbow’s aerial feats, I don’t think she would take it very well right? The moon is also honest in that it hides nothing while drawing all that attention to itself, It is not so bright that it cannot be gazed upon and the details of the moon can be seen quite clearly.
 
Not really sure how Twilight and Pinkie fit into this though. Its been hinted at that Celestia use to act a lot like Twilight in the past, and if Luna’s eventual silliness in Luna eclipsed is anything to go by maybe in the past she acted a lot more like Pinkie.

…Huh, this actually makes sense, personality-wise.

I just thought they had them. No real evidence to suggest they actually upheld them.

Yes she failed at being loyal but she always wanted to stay beside her sister. And besides all of the mane six failed at their element at least one time.

@Philosopher Stone  

@Philosopher Stone  
Luna didn’t prove to be exactly loyal.

@Background Pony #DE0C  
That’s what makes it all the more tragic.

Honesty and loyalty is very fitting for Luna but I’m not sure for laughter. But Twilight said “Pinkie Pie, who banished fear by giggling in the face of danger…” so maybe that’s why. :D

My head canon always was that celestia controled magic, laughter and loyalty and Luna Kindness, honesty and generosity, but in the flashback it didnt look like they had a conection like the mane 6

I mean, if I did, Luna would be the more sane and moral of the two.

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Luna failed at the Loyalty part though.

I just imagined them having the Elements and not representing them.

I wouldn’t take the setup quite so literally.

luna has Aj and dashie so luna is best :3