For me Transistor is the game of the year. It's fantastic in nearly everyway, and much better than Bastion.
I'd love to hear your thoughts though, if you wanted to share them? Why did you feel that Transistor was a letdown?
Narration
Supergiant seems to want to continue this "telling a story through one person's point of view and actions" that they started with the Narrator concept in Bastion. And while I could see that people might get annoyed by the narrator's voice or style in Bastion, I liked him and felt like I was actually being told a story.
With Transistor, not only is the story lacking (IMO, I guess?), but the method of delivery through one sided conversation had more than worn out it's welcome by the end of the game. Listening to the sword drone on and on did nothing less than annoy me and make me wish he would shut up already because he wasn't adding anything to the story.
Story
As I said above, the story in Transistor is something I found to be lacking compared to Bastion; the removal of a choice, the NG+ not being tied in as nicely, etc. I did like how the abilities were characters and it was both a positive and a negative that you had to continually use the abilities to unlock the full story on them... but in the end, I much preferred everything in Bastion in terms of story and delivery to Transistor.
Gameplay
Transistor borrows heavily from Bastion in that it works pretty much the same way -- you have a bunch of abilities you gain and can equip, you can upgrade them throughout the story, and you can get mutators to make the combat more challenging. However, Bastion's strength was the variety of the different weapons, from the machete to the spear to the rifle to pistols to RPG, etc. Transistor is kind of limited in that everything must stem from the sword, so the variety is already not as complex... but my main problem was that I wanted to play the game as an ARPG, much like Bastion was and in my experience the game just doesn't
WORK when you ignore the turn based aspects of the game. I've tried a variety of skills and such to make it work, but there's just something off about trying that way and I don't feel like it works. For me, it's kind of like VATS in Fallout 3/Fallout NV; you could try to play the game without ever using those, but the game wasn't designed to be a FPS -- a lot of aspects of the game sort of rely on making use of the VATS system, even if it's mostly just a "pause the action and try to get a headshot because the main design focus of this game wasn't centered around FPS gameplay".
The main plus for Transistor was the art and the music... and while those are preferences based on personal taste and extremely subjective in nature, I found both of those to be lacking in comparison to Bastion as well.
For me, Transistor is an average game that would be fine on it's own, but after the wait for something new from Supergiant after the brilliance of Bastion, it's a giant letdown.