The essence of Twilight's character was incompatible with the switch to being an alicorn. Her character was that of the awkward outsider, the introvert, the somewhat arrogant intellectual who considered herself above it all. The whole gist of the show was her being sent to Ponyville to learn the lessons of friendship. Then Faust left and Hasbro seemed to decide they needed a new toy to sell so they slapped wings on her and made her a princess too. Now she couldn't be the outsider anymore.... she couldn't be the one learning the lesson... Now she was basically 'in charge.' In one fell swoop Hasbro/DHX eliminated the character of Celestia and replaced her with Twilight. It was not only unnecessary, but it always felt awkward because Twilight isn't the wise sage type. That's not who she is supposed to be. (Then even later on they tried to re-do her character in that of Starlight. I like Starlight, but again she was not really necessary and was just more evidence of the writers struggling to maintain the quality and cohesion of the show).
Speaking of ol' Sol Invictus and Luna, the move to elevate Twi to their ranks really laid bare how little Celestia and Luna really meant in the lore. Prior to that the alicorns were this kind of mysterious, noble thing.... demi gods who everyone revered. You never got to see exactly what they were doing when they were off-screen, but your mind could fill in the blanks and invent all kinds of awesomeness. Now we could see exactly what it mean to be an alicorn through Twilight, and it was basically 'not a whole lot.' Unicorn with wings, basically. It didn't help that the writers continually had Celestia and Luna get their flanks handed to them by every villain who came along.... really takes the mystique out of them. I digress..
There are plenty of latter episodes that are spectacular, though, but yes - i do miss the first two seasons with the characters writing letters to Celestia talking about what they learned, and when the world still felt small and magical.
As for Un1k0rn....he's posted in here.