Prior win8 drivers from AMD/ATI were just the driver's themselves, without the files used for video encoding/decoding acceleration. The 12.8 drivers are basically a certified version of the latest leaked beta drivers [ http://www.mediafire.com/?1i1dja7mez7xb3a ] but missing atimpenc.dll & atixcode.dll (+ the 64 bit versions if running 64 bit Windows). Since the 12.8 drivers are intended for win8 as well as Vista/win7, it's probably fair to assume they were giving problems in win8. Those now missing files are normally in Program Files & Program FIles (x86) \Common Files \ATI Technologies \Multimedia, along with 4 other encoding/decoding files that are still present in 12.8. The separate Avivo install or setup app now includes just a streaming app for systems, mostly laptops using AMD APUs.
I don't know if Avivo is gone from now on, or if it'll be back. I can't say what the effects will be, though any effects will likely vary a lot depending on your system & what software you use. For video playback & encoding, besides the CPU software can use generic Direct X Video Acceleration [DXVA], OpenCL, &/or ATI's tech that they previously called Avivo. ATI normally uses a separate chip for video processing [encoding/decoding] on a lot of their cards, & the amount of assist it offers varies with the model of graphics card. Obviously if you use something that relies on those now missing files for increased speed, it can't if they're missing. Another concern for the future is that installing the 12.8 drivers does not remove existing copies of those files. Somewhere down the line they could be incompatible & cause problems with newer driver versions.
Unfortunately predicting that sort of thing is AFAIK near impossible because those files in Common Files \ATI Technologies \Multimedia interact with the drivers rather than just operating in isolation. All I can hope to do is give a heads up of one more place to look if someone has problems -- hopefully no one will. I'd like to say all those files are irrelevant, but everything's so intertwined with ATI's graphics drivers I can't -- over the years I've just seen too much stuff work [or not] in ways that were unexpected...
[My latest example of the unexplained/unexpected is with Virtu... Virtu is software that lets people with some Intel systems use the CPU's built-in graphics processor along with a separate graphics card -- the idea is to avoid the graphics card's power consumption when it's unneeded. There's also another mode to let you keep your graphics card as primary, but access the CPU's graphics processor for it's video acceleration features, i.e. Quick Sync. Vegas Pro 11 encodes faster with Virtu on [my ATI graphics card is primary], & it encodes mpg2 faster if OpenCL is installed for the CPU's graphics processor, yet every monitoring app shows that during encoding the CPU's graphics processor is completely inactive. Further, graphics processor acceleration is available in Vegas Pro only for ATI & Nvidia -- not Intel. I have no idea why it works that way, but it does.]