OK, so this may not be exciting to lots of people, but I think you're doing something interesting!
Firstly, there is room for an intermediary in this space. People who write email newsletters want an audience, and they pay for that through direct site advertising. If you can reach new audiences, or the same audience more cost-effectively, you have a value proposition.
Secondly, for some part of the general market, email newsletters are of interest. You seem to be offering these people
- Discoverability - I can come to you and find things I want
- Qualification - I can view samples and decide if I want them
- Simplification - your site helps me to register
Those things have generic value, so if you can access those people you can in theory gain market share. But...
At the moment, your site is failing some basic tests.
The benefits of signing up aren't clear
There's no social promotion of the site or of content you're curating
You don't let me sign up with an existing common id (facebook, twitter, gmail etc)
The ability to sign up isn't all that visible and...
...it's totally lacking at moments of truth (e.g. clicking 'subscribe)
You're over-promising and under-delivering in each of those areas
Discoverability - there are too many subcategories with one or two entries
Qualification - where are those samples?
Simplification - where is it? I'm not even seeing forms partially pre-populated, and there are very few one click subscribes.
Where is the minimum viable product? For me, you'd do well to clear both of those lists. But given your investment of time and money to date, I'd focus on (a) part-automating the list sign-up process, so that by coming through your site I'll always find my name, email address (and confirmation of that address!) pre-populated, and (b) clearing thin subcategories, either by promoting them to the parent or by grouping them as e.g. 'Other'.
Why those things? Simplification adds value - and capturing name and email gives you a frictionless embedded sign-up process for your own site. And eliminating narrow subcategories emphasises the breadth of what's there, not the shortage of what you don't have.
Very close behind that (for me) is sample newsletters. You may have to deal with some permission issues creatively, but it seems to me that your directory is meaningful if there's a sense you've actually verified the lists. I'd value 'preview' over 'go to site' - a good email newsletter will show me what I need to see as well or better as the website it supports. And that 'preview' page can also be the beginning of the sign-up process.
Ultimately you'll be selling audiences to list managers, and analytics will be key to the initial sale and to adding value to paying customers. And if you're successful, you can grow the value to users.
If you make my sign-up consistent, for instance by guaranteeing that I use a particular email address or name, then that will help me identify these emails as I come in, and spot where my details have been sold on to some third party. That's a real convenience, and supplements rather than competes with the various browsers' attempts to automate generic form filling.
So, all that was reaction to your site and concept as they stand. But if it were me, I'd be pushing one step further.
You have bet on users finding value in managing their subscriptions. And you have bet you can monetise that directly through the email newsletter providers or indirectly via advertising etc.
As a potential user, I wonder why you don't offer me the next logical step - a dedicated @subscribist.com email address for my lists. If you sat between me and the list, you could automate the tedious email verification process, prevent unsolicited email coming in from sources I haven't solicited, let me manage the subscribe and unsubscribe process from a web site, let me terminate even emails that don't include an unsubscribe link, let me see the emails individually or in daily or weekly summaries - a whole lot of things.
I think I'd actually pay money for that.