I agree that the message could be clearer, but as pointed out by Simon "You need X rep" would be hideously unhelpful to the users who do not know what reputation is yet.
Any user who has just stumbled on the site and signed up to ask a question would likely be in this situation. A compromise might be.
Sorry, new users can't do X
until they have Y [link]reputation[/link]"
A little long, yes, and I'm open to any suggestions about shortening it. But this conveys the following information
- New users can't do what you're doing
- We have a reputation system here
- You CAN do what you want to do once you have Y reputation.
- Here's a link explaining reputation and (presumably) how to get it.
As you noticed, with only "New users can't do X" the new users are left with no indication of how to become... "old". With "You need Y reputation" new users are left in the dark about what/why reputation is.