I have two computers, one with Avast Free 2014 and one without. I had this problem only on the one with Avast 2014 after installing it. I uninstalled Avast then had to uninstall and reinstall the MS apps, then it worked again.
This solution works for some people, but not for me and many others. I tried it today with Avast uninstalled and no AV running, but my live tiles, including the Store tile, do not work. I've run the MS AppsDiagnostic several times, but to no avail.
In my case I believe it has to do with my User account on Win8/8.1. I say that because I created another Admin account just for troubleshooting, and when I logged in to that account, lo and behold--all the live tiles including the Store worked just fine.
In fact, that was the only way I was able to get the upgrade to 8.1. I was hoping the upgrade would fix the problem in my main account, but it didn't. Fortunately, I'm exclusively a desktop user at this time, so I won't miss the part of Win8.1 that doesn't
work on my computer.
Microsoft needs to come out with a fixit to solve these issues without people having to do a clean install, which for some of us would mean many hours of re-installing and re-configuring.
I sympathize with those who do use the metro interface and apps and who are still struggling with this. I thought Microsoft did a great job on Win7, but these days I'm finding it pretty hard to like Microsoft.