I think I read somewhere that you have to time your block by tapping the block button at the right time and to bash you hold in the Block button, I think its a good idea that way we don't just sit and cower behind our shield. If I find where I heard this (if I did) I'll post a link.
http://www.gameinformer.com/b/features/archive/2011/01/24/skyrim-building-better-combat.aspx
Second to last paragraph.
A good offense must be accompanied by a good defense. To make defending a less passive activity, Bethesda has switched to a timing based blocking system that requires players to actively raise their shields to take the brunt of the attack. If you hold down the block button, your character will attempt to execute a bash move.