A lot of intermediary programs might hijack <F1>. I know that GNOME Terminal hijacks that key, and I think GNOME Shell does as well.
NERDTree uses '?' for displaying help options in its pane: http://cdn.tutsplus.com/net/uploads/legacy/951_vimEasyMotion/nerdtreeSS.jpg
Nobody hijacks the '?' key and it doesn't trigger a special action in Tagbar either.
Add ? as help key, closes #194
Okay, I've added ? as an alternative default help key.