Spiral Knights has been given favourable mention (twice!) on penny arcade. And the maturity level of the average player is rather high. Case in point, i rather ripped off a certain fellow in a trade deal - he wasn't quite aware of the value of what i was selling. Realising his mistake, he sent me a nicely worded letter asking if he could have his money back. It was beautifully worded, capitalized, punctuated. A full paragraph of text without the word "faggot" in it would do, but no, this was panacea. Of course he still didn't get his money back, but i thanked him politely for his letter.
In fact in my 36 hours(!) playing the game, i haven't seen the language filter be used ONCE. That's a whole lot of people not swearing. I'd guess, based on this, that right now your average player is a middle class 20-something (Probably quite a few university students). And here's the point: These people have disposable income.
Please, don't advertise on Neopets again. (It may or may not have worked for Bang! Howdy, but if you go that route, be prepared to lose the majority of the players you currently have. If people wanted to be surrounded by screaming idiots, they would work for EA)
Now, as you have this rather unusual player base, you can do a few things that other multiplayer games can't afford to.
1. Requiring Higher Levels of Co-operation.
Right now i can solo to roughly the same depth as i can co-operatively. The downside is that i get less dps, the upside is that i get more of those oh-so-important health capsules. It just takes longer. If you want to add enemies that really require a two-player concerted effort (Say, ones that can only be dealt damage from behind), the player base can handle it.
2. Co-operative Puzzles.
I get the feeling these have already been suggested and vetoed internally. I urge you to reconsider that decision - players can and do communicate in dungeons.
3. You can make direct appeals to the player base.
TRD can still be considered an "indie" game company - which has some currency with your current audience.
For instance, while i would never buy a premium item from one of those nasty Korean companies,
i would consider buying one from you because of your "indie" status.
4. You can make the game a little nastier/ less forgiving/more competitive.
I realise this is something you've been working hard to avoid thus far, but just be aware that the player base can handle it if you do choose to go that route.
5. You can have better writing
Right now what little speech there is designed for clarity and little else. But if you want to have something a little more subtle, perhaps flesh out the plot, the option is there.
Um...this game was designed to be a simplistic affair at its heart. So no.
Maybe a yes on the cooperative puzzles.
Also you start this off with saying you ripped someone off? Way to go...