Okay I seen the S4 finale as soon as it aired. and I've known of the Rainbow Power much earlier (as I'm sure many bronies have) And as I found 2 cents yesterday, I'm gonna throw them in.
To start a major point of contention are the colored streaks in the manes. As a guy who like colors and color theory I'm gonna focus on them.'
Twilight Sparkle: She gets a yellow streak. As it is Yellow is a complement of Purple. And her primary color scheme is over all "dark". However her name refers to the meeting place of day and light. And yellow is ofthen associated with the sun. So the yellow streak would refer to the sun, and by extension her mentor, Princess Celestia.
Pinkie Pie: Her steak of Yellow and Cyan could refer to the CMYK color model. at first glance that might seem at odds since hat refer to ink, then printers, then dull office settings. However these 3 are very bright and vibrant colors. And are actually the Primary colors used in blending (though Magenta and Cyan are confused for Red and Blue respectable, do to them technically being shades) The "K" part is black and Pinkie had black streak in the toy line (not canon but still worth mentioning). There also seems to be an orange streak as well. Whick along with yellow goes back to the sun symbolism, this this time referencing her perpetual optimism and happiness
Rainbow Dash: No new colors as She already has all the colors of the Rainbow (and Magenta doesn't actually exist within the visible light spectrum or the Rainbows)
Applejack: The red should be obvious being the most common and stereotypical color of apples
Fluttershy: Her tealish streaks could refer to the forest (where most of the animals she tends are associated with) or spring
Rarity: Like Pinkie, her streak might also refer to the CMYK model (again sans black). However the fashion trends change all the time and with them what colors are "in style" so This might be a more overt reference to color blending or the "is the new Black" saying as she tries to be a trendsetter rather than a follower. A Shepard instead of one of the Sheeple. Colors are very important in art, which fashion and haute couture are a part of.
Another noticeable change are the wings. They have ceased the monochrome aspect and Incorporated the some of the mane colors. Twilight's now have some subtle pinks leading to the purple tips. Fluttershy has teal leading to pink and Rainbow Dash's are the Primary colors we know (Red Blue and Yellow) with purple edges
As for the manes. Well for those who actually watched the finale, you'd noticed that Twilight's fight with Tirek has a lot of DBZ vibes. I think the poofiness are yet another homage to DBZ as well as the original MLP toys (Which are made in the 80s if one recalls)
And so I end my reasoning for Hasbro's changes. While I say that I like them a little bit I will admit that they could have been handle better.
In other news. I Have gotten a 3ds recently and gotten Rune Factory 4 as well as playing Bravely default.