What about a 2.5 year old account with no karma? You'd be surprised at how many lurkers there are. I know quite a few (and was one myself up until a short while ago).
EDIT: added emphasis
EDIT: added emphasis
But the bar is much lower than that. For the purpose of just keeping spam accounts out, I think the combination of a karma threshold with average non-karma accruing activity over time (such as upvoting others, clicking through on articles, etc) could be an effective filter.
I'm usually afraid to say something wrong, since I know that there are always experts on the topic of the post. It makes more sense to me to let said experts explain a particular topic than to possibly disseminate wrong information, even though I might have a useful perspective to add.
The issue is if people that see that you're wrong, instead of correcting you, just downvote you. In my opinion, a wrong statement should not be buried with downvotes, it should stay at 1 point with a reply that corrects it and that get a lot of upvotes.