1) Professional support
While Comunity version of magento is free like wordpress there is a well financed crew of core developers looking after the source code. Some purest will argue open source is best, but open source often ends up being a cluster of bandaides and work arounds. While no software is free of these you tend to find a lot less of them in software with paid paid employees. Additionally they it is part of Ebay/Paypal group, which are well financed.
Most developers have know idea how refreshing it is to work in Magento If a client wants something made, changed no problem. It doesn't matter how exotic It can be done, we should know we have made a few of them. You don't find people often looking at a magento store and say 'oh yeah a magento store, I can tell it looks like all the others'.
3) Good selection of extensions/plugins
There some really good extensions available for magento. Most are very business focused and based around running a store. Magento is hardcore business and there are the hardcore business extensions for it.
Out of the box Magento can handle some huge numbers; 1000 categories, 100,000+ products, 10,000+ of users. That means an owner can get into the market for a similar cost to a small professionally setup wordpress site, but stay using the same software as they grow to a large size. It is possible to push the boundaries further with some tricks.
5) Product Search
Product search out of the box is great. Its fast, doesn't get bogged down. With a little further help from a theme or extension you can create a very powerful search tool that users will love.
SEO is the make or break of a website. Poor or not ideal SEO ability out of the box will cost you again and again. All the meta fields are right there for everything, no extension needed. You can write your own urls and site nav is clean. Google friendly sitemap is all there automatically too.
This goes back to number one. Because Magento is well backed, the big players have invested in good software for integration. Some fantastic examples are; myob, quickbooks, facebook store, shipping, paypal pro and many more have professional well built extensions and often have support that will install them for you or atleast test them with you or your development team. We have an ongoing relationship with Paypal Australia which is awesome. The money it saves clients is in the thousands. For example imagine you do an update and your Paypal gate way goes down, this might take you or your developer hours, days a week to fix. Not for us we just call or Paypal developer support team member at home, if need be, then he helps fix it for us! I've never heard of that with wordpress. I have had similar experiences with most other extensions. FYI a lot of stuff is already built in and only requires some configuration.
Magento out of the box has some nice tools and quirkes to help with remarketing. One of the nicer ones is the newsletter feature. With this feature you can create your own txt or html newsletter and then email it to all those customers who have purchased from you. It also has a nicely advanced promo code creator, instore specials creator with set and forget, some cart abandonment details capture quirkes (not really a feature). Also it has some hand customer groups creation settings. Some cheap extensions can automate this process and make it fly.
9) Import and Export of products and Customers
I must admit the first time you try this you want to punch Magento in the face, but once you have it dialed in you're away. Export you stuff to .csv file ammended it and upload it. It makes managing mega stores and even 30 product stores so much easier. LOl, I have actually had to enter by hand over 60,000 products before, never again I tell you. Oh and customer management is a piece of cake...how easy is it to export clients to .csv and do anything with it, such as; mail out, emails, use mailchimp or similar. If you are converting to Magento you can import all your clients from your other system, which maybe be a nightmare. However atleast getting them loaded is easy and the end of it.
This is an advanced feature, but it means that magento can talk to other apps, software and services and share details via a secure link! This makes magento near limitless with what you can do. It is by far my favourite Magento feature and mebsites has made its name with this feature.
11) Magento Caching
Did I say 10 I meant 11. Again out of the box Magento comes with some nice caching features and compression features. You can further enhance these with some elite server setups (see mebsites) and you have a bullet of a website. Where Wordspress struggles to load in 5-20 seconds even a huge Magento store can load in sub .5 and sub 1 seconds with ease. We all know customers leave or never arrive when your store is a slow pig.