Note: Copied and pasted from here and edited.

1. Rainbow Roadtrip took quite sometime before the plot actually began to move. Until Mayor Skies sung about Hope Hollow's descent into despair, it spent a great deal of time showing what went on inside that "luxury resort," the ponies' behavior, and the mayor's façade when leading the tour. But all of this took about one-third of the 60-minute runtime. That's way too long, and the lack of humor makes this plot develop bland and way too simple.
2. The songs aren't the best of the series or up to par of Best Gift Ever's introduction. The lyrics and melody feel unfinished, clumsy, and lack the rhythm.
3. In comparison to the movie or BGE, Rainbow Roadtrip's much more mellow, and that was the point. However, say what you wish about the former outings; they weren't boring, and all the action keeps the audience engaged. By making RR's plot so simple, the mellow direction makes this story very bland.
4. Collectively, the last two seasons have some of the best dialogue of the series. Another couple of rounds of editing would've helped tighten it and give it some more humor.

But it has its own strengths.

1. It's competent. The movie ignored a world of continuity to connect the story, mainly Twilight's inability to possess the staff through her magic or teleport, and even the thought of Discord or Shining Armor existing. In BGE, Fluttershy's intelligence was zapped so Flim and Flam could get away with their scheme for a few more hours. Here, despite no stakes at any point, it doesn't contain any big mistakes, and it especially not an ableist post office scene. Every action makes sense. When Twilight needed to teleport, she did. When Fluttershy needed to help acquaint ponies, she did.
2. Everyone's in character and not basic at all. Already wrote this before, but I'll write it again. Applejack and FS were merely there in the movie, and neither Dash, Spike, nor Rarity did much. Twilight and Pinkie were easily the most complete with the former taking over most of the spotlight. (The movie had the M6 saving the day, but Twilight had one of the two biggest character arcs.) RR balances all six much more naturally, using their best strengths to help revive hope within Hope Hollow. Early on, the characters show their flaws, like Dash's ego and Twilight's perfectionism. After Mayor Sunny Skies explained what happened to Hope Hollow, they show why they bear the Elements of Harmony. Sometimes their methods to help Hope Hollow regain hope took a little more time, but they remained patient throughout, convinced their lessons will help them.
3. There's no antagonist, and do we need one? Nope! Everyone is grumpy, because they feel Hope Hollow won't return to its glory days. The fact that there's no Rainbow Festival to cheer ponies up increased the hopelessness and despair of the whole town. Mayor Sunny Skies also felt responsible for causing the town to lose its color and ruining the legacy his family left behind for him, even though Petunia tried tirelessly to convince him otherwise. Because he felt so guilty over his supposed accident, you can't help but root for him and the M6's quest to revive the festival so he can let it go (no pun intended ).
4. Its biggest strength: It's so wholesome. Hope Hollow's split and bitter, and after he told his story, they worked to strengthen each others' relations. Their methods to help them and kindness were genuine, and they really worked with everyone to help them recreate the Rainbow Festival's magic.
   
   Like Rarity herself, Kerfuffle's a fashion designer, but because of Hope's hopeless magic, she now feels shy about exposing her creations with others, fearing rejection and lack of appreciation. With the Rainbow Festival's revival, Rarity gave her the confidence and guidance needed to showcase her talents and earn that respect.
   
   Dash realized Barley and Pickle had trouble flying, so she gradually trained them.
   
   Twilight worked with Petunia to find a spell powerful enough to repel the gloomy magic blanketing the town.
   
   Torque Wrench felt unappreciated working as the mechanic and carpenter of Hope Hollow, working out of necessity. But Applejack, understanding how it takes a lot of understanding the craft to fix more than just the billboard, gave her the needed confidence to rebuild the rainbow generator.
   
   The Hoofingtons and Moody Root were long-time neighbors, but didn't get along, and Moody refused to share his apricots with them. But Pinkie's happy-go-lucky charm and Fluttershy's openness to others created a bridge for them to communicate and exchange.
   
   It's charming, heartfelt, and warm, and you feel so happy to see them succeed.

This special's a very pleasant surprise. I didn't expect much, but I'm glad to be wrong. Between the film, BGE, and this, I may prefer the film. But which of the three is the best? My vote goes to Rainbow Roadtrip. And it's Kim Beyer-Johnson's best episode so far.