Is it possible that the words in the ad you use to bring the traffic can have a negative or positive effect on conversions?
Without a doubt. While the on-site experience closes the deal, the ad is your primary driver for encouraging the *right* type of visitor to your site. The language you use (whether it be informational, sales-focused, inclusive or restrictive, etc.), the features that you promote, whether or not you include a promotion or the price of the item or service in your ad can alter the conversion rate significantly.
If so, would it be significant enough to be concerned about?
Well, it's certainly an opportunity that I would take advantage of.
For instance, including a price for a product in your ad will deter users who want something cheaper from clicking - thus improving conversion rate and decreasing your costs.
Dave