Here's a few reasons I sometimes miss great designs.
It's amazingly difficult to look past a colour you don't like. This is a gorgeous all over cable design. Look at those fantastic edges. I hate that colour on me it makes me look sick. If you can't see past a colour look at project pages to help train your eye to imagine it in a different colour.
If the sample and the model are mismatched you might miss a great design.
Now go look at this project page, do you like the pattern more now?
Some knitters skip right past patterns photographed on mannequins. Most indie designers can't afford to use professional models. See Poor Fit above as to why using friends is not always an option. For more on Model vs. Mannequin - Which do you Prefer? see this post.
I have a different perspective, when I see something on a model the information I get is how it fits her not how it will fit or flatter me. This was confirmed when a knitter brought me a pattern and asked me to help her fix the shoulders based on the photo. The first step should have been to compare the schematic measurements of her chosen size to her actual size. We are heavily influenced by visuals unfortunately what we see is not always what we really get. Tall skinny models tell you how things look on tall skinny models not how they look on you.
If you are thinking shawl you most likely won't see a gorgeous pullover.
Lack of imagination
Great basic patterns that fit you well can be easy to add additional design details to. Look at the project pages for other ideas. Try to imagine the item knit in a different colour or in your favourite yarn.
Here's what Twist Collective does to help you imagine the garment in a different colour.
You only Looked at One Photo
Sometimes the primary photo never caught my eye but the second one does.
Here's an example:
See my post here on Tips for Using Ravelry when Choosing Patterns