Many of the obstacles invoved in shopping on-line will change over time as the technology and norms change but here are a few of the reasons people say they choose NOT to shop online.

Anxiety 
This has clearly been something that has changed over time but is still an issue for some people. The idea of getting your credit card stolen after putting the information on-line is a serious concern for some. Even if users trust the store or the people at the store that have access to the credit card information some individuals worry about typing the info on the computer and not knowing who has access to this data. The fear of hackers, malware, viruses and other security concerns can be enough for some people to forgo shopping on-line entirely. This may or may not be a legitimate concern in all cases but it is something that is a real issue in the mind of some shoppers.

SIDE NOTE – In some areas of the world people would shop online only if it was COD (colect on delivery) becasue they do not trust that products will get to them.

Do not know and trust the store / things can go wrong
People may not have the same trust of an on-line store when they do not have intimate connections with it the way they do with a local brick and mortar or a larger brand retailer. They may trust rei.com because they trust REI the brick and mortar store. The same individual may not trust an unknown sporting good store on-line the same way, even if they have not shopped at REI.com or the smaller on-line store. The brand experience or perception can translate from the traditional store to the e-commerce store.

We have more chances for things to go wrong when shopping on-line.
Will it get delivered?
Will the item have a defect?
If not, can I return it?
What if they send me the wrong color or size?
Do I have to pay return shipping?
Do I need to go to the post office and stand in line?
Do I have a restocking fee?

“I could have just gone to a local store but now I have to deal with this hassle.”

Many larger online stores that have a brick and mortar presence do not have to deal with all of these worries as the shopper has experience with them or know they have a brand and reputations to withhold and a physical place to go and fix any problems.

Need the product soon
If you need the product today you may not choose to shop on-line. Some products you want or need right now. You may need the product right away, say for a birthday party later that day, or just want the instant gratification the product gives. Either way, this can be a reason to shop at a close physical store.

Side Note –
This is clearly something amazon is taking into consideration when they are aiming for 1 or 2 hour delivery in some areas with select products. They realize that not all, but some products are needed right away and the 1 or 2 day delivery will not cut it for these situations.

Do not like getting deliveries
For some people getting deliveries can be pain. A box sitting on your front porch does not work in all neighborhoods or for all products. If you have a valuable product sitting on your front steps the anxiety about them getting stolen is too much for many people. Other individuals have to go to the front desk of their apartment complex or must be home to sign for a package.

Cannot see or try on the product in person
Seeing what you are getting in person can make a huge difference for many items people want. Does the clothing fit, is this the right color, what is the fabric like. Is this chair comfortable to sit in, what conditions is this used jacket really in? This will depend on the person and the type of item. If you work at an on-line store that sells these types of products it can be challenging to answer all of these concerns people have.

Every reason that people do not currently buy on-line is an opportunity for innovative stores to fix or eliminate the roadblocks and create new and better shopping experiences.