You remember how your mother told you not to judge a book by it's cover? Well... it doesn't hold up as much as we would like to think. People are constantly making judgements, often times within seconds of first exposure to something or someone. They do this both consciously and unconsciously.
It's part of how our brain works. What we see is automatically filtered through a series of brain cells to recognize associations that identify threats and rewards. Obviously, poor design is not going to be threatening to you, but the brain analyzes the information the same way.
When it involves minor things (like poor spacing or overcrowded text), we may not cognitively be able to put words to why we feel something. People just "feel" like something is off or they just love it, without being able to say why. If it is a big thing (like someones face being cut off unintentionally), people are more likely to put words to it. Negative feelings are barriers for your potential clients.
One of the major factors that contribute to these "feelings" is confusion. If you say you are a professional service, but your logo looks pixelated, jagged where it should be smooth, generic like clipart, too busy, too basic, etc., you introduce confusion. If you talk about how simple your service is to use, but part of using your service is having to read a lot of instructions that is bunched together in one big text paragraph, you introduce confusion. If you say your primary differentiator is caring about your clients and it is difficult to find your contact information or get ahold of you, you introduce confusion.
Confusion will kill sales before they even get started. For others, these feelings will play out during the sales process. It will cause them to move forward with you cautiously. It puts your clients on the defensive. These are all barriers that will need to be overcome to get a sale. Even the clients that move forward with barriers present are more likely to look for confirmation of their suspicion.
We have all had clients that seem like they are dead set on finding what we did wrong or are constantly trying to sabotage their own experience. It is partly because they are. They are looking for the negative because it is what helps them make sense of why they feel like there are barriers to working with you.