We've known that the Steam Controller will see quite a bit of tinkering thanks to an interview with a product designer who worked on the controller during CES 2014 - but the Steam Dev Days has revealed a bit more about the impending changes.
Twitter user and Dev Days attendee @DaveOshry noted that beta testing has proven successful, allowing the Valve team to head back to the war room for more redesigns:
"The Steam Controller will see more changes before it goes on sale. They've gotten tons of feedback from the beta."
Various other tweeters have also stated that Valve is actually getting rid of the planned touchscreen and replacing it with physical buttons for retail versions. Twitter user @omglazerkittens (a.k.a. Becky Taylor, community manager at Signal Studios) says, "They [Valve] are experimenting with analog rather than track pads."
She later wrote "[T]hey said that there is a possibility of bringing [the touchscreen] back in, but for now they don't want it."
Change is good
Changes to button placement were mentioned during CES and Dev Days attendees seem to have confirmed Valve is working on d-pad and standard A/B/X/Y configurations.
Again, @omglazerkittens tweeted, "They will be bringing button diamonds for backwards compatibility with other controllers. No mid touchscreen."
Other attendees mentioned a feature called "ghost mode." Early demos shown to The Verge point to ghost mode projecting a ghostly image of users' thumbs on their TV screens as it passes over the touchpad. Users can then press virtual buttons on the screen.
@omglazerkittens also noted that users will have the option to use rechargeable batteries with the Controller.
On the developer side of things, the controller API will also support up to 16 players at once.