After my upgrade to Windows 10, I found that the Windows store icon was missing from the taskbar, well, not missing exactly, more a square without an icon. It launched fine but the taskbar icon wasn't there.
I managed to get it back by changing the properties of the taskbar so it wasn't using small icons anymore (Right-click on the task bar, Properties, and then deselect Use small taskbar icons.
I then removed the Store icon by Right-click and then selecting Unpin this program from task bar.
I used search to find the Store application, and then Right-click, and then finally Pin to taskbar.
I can now have my taskbar using small or large icons and the Store icon is still visible.