[user104658]

The problem is that not everyone has photo ID so, unless you implement mandatory citizen ID cards (which I am not in favour of) you're making it harder to vote for a section of society... And of course, it's certain demographics (the least well off, those who have just reached adulthood, and the elderly) who are least likely to have valid photo ID, skewing the vote.

And even people with ID, imagine not being able to vote because you've misplaced your passport? Or you dropped your wallet with your driving license on the way to the polling station? It's really not ideal.