Thinking in terms of how to help a new viewer of your virtual community become interested enough to continue reading, return, and become motivated to react or even contribute content is the way to approach this question.
Typical social media factors would apply here. What's an optimal percentage for your community depends on the nature of your audience, and as you mentioned, the features of the actual experience. But think of it in terms of, for every 100 viewers (consumers) of content, around 10 will react (engage), and 1 will contribute. Of the 1 out of 100 (or so) that contribute, 10% of those will contribute regularly, basically becoming your super members.
Now you mentioned the expected topic areas, and thus implying the audience, but you did not mention the purpose of the community. Why does your community exist, and why would someone of your target audience want to join your community? Nailing this question is the most important part, and then understanding the nature and preferences of your intended audience is the second most important part.
For example, I help manage several Apache Software Foundation communities. The purpose of these communities is to collaboratively build open source software, and help the users of this software. We use a variety of old school, disconnected tools for collaboration and communication such as email distribution lists, git, Jira, and a wiki. Its the preferred way that a lot of our typical contributors and users want to interact with each other, and they are familiar with the tools.
So, after designing for your purpose, and your intended audience, here are other factors that will matter in creating a more engaging experience so that you can reach your optimal engagement and contributor ratios:
1. Content
Good content is key to an online digital community. You won't get the viewers, and you certainly won't get them to return or motivate them to engage if you don't have good content to start with.
2. A corps of dedicated super users and experts
This is partly where your good content comes from, and its with these people that new users will be motivated to interact. If this is a new community, I wouldn't go live without a commitment from 10 to 20 expert folks to post interesting, relevant, and thought provoking content on a regular basis, and to come to the site frequently and interact with content posted by the other members of your starting super user committee. These people do not need to be moderators or have any other responsibility beyond acting like the engaged, contributing super-user you seek to have more of.
3. Social interaction features that allow members to show creativity and personality. I realize this is probably the crux of your original question, but I'm going to avoid a laundry list of features. You can look at any social media service, or forum or community software and get a whole bunch of feature ideas. There's benefits and drawbacks to all, and what and how many you choose really don't matter so much as long as you have one, or a few ways to interact.
Similarly, "likes" and votes are a form of engagement, but from a community building perspective, they do little to create contributors, which should be the most important goal of building your community, unless what you are really discussing is an ad-driven media site.
4. Ways to serve relevant content to viewers quickly
This would include trying to guess what a viewer might be interested in. In advanced cases, maybe you can guess a topic that drew them in, and you can suggest similar articles related to the topic they're reading. You would want to make the content searchable, and make use of strong meta information such as topic and relevance tagging, author linking, and trying to guess the nature of the users context. For example, for new users, they might come to the site as a result of a google search, or response to a social media post. The topic relevance will suggest other articles, and the experience will also ask them to log in to react to a post if they are an existing member, or find out how they can learn more about their topic of interest by becoming a member and creating a login.
As an example, I recently saw that 75% of Netflix views came from recommendations of the system to the viewer.
5. Communications
Once you have contact information from your members, the best way to get them to return is to make sure they see what they're missing by not being logged into the site. In fact, in the case of the members of my communities, they don't want to be logged into a site. They'd prefer to just interact with each other over email.
You'll want to allow configurability of outbound communications so that users can decide how much "chatter" they'd want to see in their email, or text message, or social media feeds, or however you might choose to communicate. A reasonable default would be a daily digest of what's new and hot, and a weekly curated newsletter.
6. Mechanisms and ways to deal with bad experience problem-makers
This would include spam, which is a never-ending issue for online forums, and trolls. These are well covered in other questions, but just know that you should plan to have a way to minimize and deal with these problems quickly when they arise. If not, this will cause your engaged members to become disengaged, and cause others to never return.
If you have an answer for all these factors: know and design for your purpose; know and design for your audience, and follow the 6 general practices, you will create an engaging site.
As far as coming up with compelling unique ideas, I'd tell you to look at your purpose, your audience, and your content to find the unique and compelling idea. For example: online editing of videos makes sense for youtube, but it would be silly for The Apache Software Foundation. The ability to annotate comments on a soundtrack makes sense for SoundCloud but will go unused on Yelp.
