What are some things to take into considerations when choosing a shopping cart??

How user-friendly it is to the end-user and you. Does it fit your needs, and if it's a clean design. I would recommend WHMCS if this is for hosting.
All the best,

How customizable the shopping cart is and the features of the shopping cart. Make sure the cart has all the features you want.

It is a good idea to think of your customer when searching out a shopping cart / eCommerce provider and how to design your website.
For example, some consumers would be more likely to use alternative payment methods (such as paypal or google checkout) while others may want something different. Some shoppers would be incented to buy if they had a "best offer" option or "wish lists". Of course not all of these described features will be relavent to your customers, which will be driven by the types of products and services you sell. You can easily get wrapped up with features, so pick out the top 5 - 10 that will really make a difference then ask potential providers about how they do them very well.
As you grow, other aspects may be worth investigating such as percentage transaction fees charged by the eCommerce provider, automation of shipping and warehouse processes, and marketing and promotions.
Good luck!

If you are looking for open source shopping carts, I recommend you to have a look at Magento Commerce, OsCommerce, ZenCart and CubeCart. If you need something powerful and customizable then you should definitely take a look at Magento. :agree:

The cart should be seo friendly and the back end/admin area should well-designed and easy to use.

A good way to narrow the field is deciding what kind of cart is most appropriate for your needs: open source or proprietary .
Open source carts have come a long way. The latest offer pretty much everything you'd find in a proprietary solution, provided you have the basic technical expertise to install and configure them. Most have become much easier to install than previous offerings and include numerous developer contributions for increased features and customization. Technical support tends to come in the form of user forums which, at least for the most popular products, are heavily trafficked and often yield answers in a matter of minutes.
Which of these (or other) open source products would be best for a particular merchant's online store? That depends on individual business needs. Factors like design flexibility, search engine friendliness, reporting and back-end functionality are important to weigh when choosing between products. Most product sites include demos and lists of live sites running the software, which can also help differentiate between competitors.
If you are uncomfortable with the technical requirements of setting up a shopping cart application, It's possible that open source solutions are not for you.
Proprietary programs cost money, but generally offer configuration, design and support services that can take the headache out of launching a store for the less technically inclined.
Open source carts to consider are ZenCart, OsCommerce, Ubercart, and some proprietary carts to look at are Shopsite, Nexternal, Volusion, 1shoppingcart. MivaMerchant is another popular cart.
If you are currently processing credit cards, ask your processor which carts they integrate/recommend, they may be able to assist you in setting up your e-store.
Happy shopping cart shopping!

To give your a bit more precise answer from an experienced guy... OsCommerce is really the way to go. It is simple to use (once you've spend a few hours getting an overview on the whole) and is REALLY advanced since people keep submitting updates to the system (engine/framework)
There's a million different templates so your shop don't have to look like the 10000 rest.
Anyway OsCommerce is highly recommended!

What are some things to take into considerations when choosing a shopping cart??
There's many but a few I've stumped upon and being a strong point from clients are
1)Reliability -
2)Price + supported Payment methods
3)Testdrive
4)ease of upgradability + SEO capability
5)ease of designing the layout, or going live with minimal programming skills
6)lastly, great support from either the Hosting Company or the actual Vendor.
Note that a hosting company should give you good general blanket support, such as installation, ongoing upkeep, but nothing more. Choose a company you can actually "call" if the need to call will spring up.