Hello! I just installed i3-wm, because I have a similar problem with the Shift key in scrotwm. I set alt as mod key. And guess... I can neither shutdown or restart i3. Some bindings are working, some not (for example mod+Shift+num for moving a window to another workspace) - similar to my scrotwm problem, all actions involving the Shift key.
I don't know what is causing this problem. All you sure other actions involving the use of Shift key are working for you?
p.s. I'm happy I tried i3-wm, I simply adore the tabbed layout, I feel like I'll stay with this wm