When selecting a web design agency for your website project, apart from looking at their portfolio and reviewing their client testimonials, the following ten topics are a must to include in your checklist:

1. Content Management System (CMS) - open source or commercial. Commercial CMSs would look you into a contract with maintenance fees. Best open source CMSs are Joomla and Drupal. WordPress is still suitable for simple websites for the most part. Read more about differences between these CMSs in this blog post. Most open source CMS are easy to use and do not require a lot of training. They also allow users to manage and update content on their website without the help of a web developer. However, you need to keep them up to date. With Joomla, for example, upgrades after version 3.0 are extremely easy to implement. For most CMSs, upgrades within a given release can be implemented with a click of a button. But this is not the case when a new release version becomes available.  Make sure you ask whether your web design agency provides regular CMS upgrades and that the terms and limitations are. 
2. Website Hosting - it is not advisable to store all eggs in one basket. It is best to select specialized companies, i.e. those who specialize either in hosting or in web design. Find out if your web design agency can recommend a trusted website hosting company. Most hosting companies look equally qualified when you read their websites, but things can be very different in practice. Find out if your web design agency will assist you with moving the website from one hosting company to another.  Sign up for a free website monitoring service (such as GTmetrix, for example).
3. Security - security issues can originate with your website and with your server. Some hosting companies provide sufficient disaster recovery services in case of a hacking attack. But some don't provide any. Find out what is included in your package. In terms of your website, find out what security measures will be put in place by your web design agency and what support you can get from them - and what the terms are - in case of a hacking attack. 
4. Speed - if you are using a web content management system (CMS), any CMS, your website will be slower to load because advanced CMSs contain a lot of files. This is normal and to be expected. However, if a website is large and contains a lot of images and if your hosting plan does not provide for optimized speed, your site may load at a sub-optimum speed. In order to remedy this, there is a number of techniques that can be implemented on your website that can improve its loading speed dramatically. Find out what is included with your website package.
5. Mobile and Tablet Technologies - find out what optimization your website agency provides for mobile devices. Make sure you understand the difference between a "custom mobile layout" and simply a "responsive" website. Both are good options, but you need to know what you are getting and for what price.
6. Website Usability - ensure that your website will have a navigation with a two-click rule. Double check that an automatically generated Site Map and a Site Search will be included. In addition, ensure that your website will contain stylish calls to action, proper paragraph headings and spacing as well as visually pleasing and easy-to-read typography (font styles) and formatting. Make sure that a website registration form is included in your package. Your copyright date needs to have a code that would update it automatically. And needless to say, your contact information needs to be easily accessible from every page with one click. Also specifically request to set all external links to open in a New Window. 
7. Website Content - find out if a web design agency has capabilities to assist you with your website content. If you are paying for website content creation, ensure that you will get "unique content", i.e. created from scratch specifically for your purposes. Search engines have evolved to detect duplicate and even partially duplicate content, which can result in your site's being downgraded in ranking. Include an obligatory spell check in your website contract.
8. Website Analytics - most websites contain a line of code generated by Google Analytics that allow you to monitor your websites' traffic and user behaviour. Google Analytics is free, all you need to do is sign up and then follow the instructions on how to generate the required piece of code, which you will later send to your web developers. In addition, larger websites often use additional analytics services that they buy from specialized software-as-a-service (SaaS) providers. Find out how your web design agency can help you interpret the analyse data and at what cost.
9. Search Engine Optimization (SEO) - usually SEO is a separate service but some website design agencies, like Joomla Bliss, for example, include basic SEO in their design packages. Find out what is included in your case. Also find out if your website design agency is qualified to provide with you with custom SEO training. 
10. Social Media Integration - integration with social media websites is a very simple task technically. But many web design agencies charge handsomely for this "value added" feature. Insist that it be included in the package. You will need to provide URL addresses for your social accounts - Facebook, LinkedIn, Twitter, and Google Plus. Those four are the leading social media outlets at the moment. Note that you do not need to disclose your passwords, only the addresses are required.  

It goes without saying that it is also important to find out what website maintenance, support and training options are available. Joomla Bliss offers free training and free post-launch support for two calendar weeks. Other web design agencies might charge additional fees for these services, so it is important to find out what is included and what is not.