So, normally, I don't even check this, but with the tmc craziness going on (if you don't know, you don't want to-really), I decided to look at forums I haven't looked at in a while; didn't expect to actually reply, as I'm usually a lirker, but:
As soon as you abandon telnet, you abandon blind players. Mostly. The thing is, most people don't know how to make an accessible web client, and I'm not even sure this can be done (yet--we'll have to wait and see how well screen readers support html5). We're a small part of the mudding population, but, as I am a part of that population, I think it's worth considering. That said, so long as the game is room-based, making a desktop client (c/c++/java/etc) isn't so bad, as there's apis that can be called to send information to the screen reader (this is what mushclient does atm). To be honest, even if it isn't room-based, given certain constraints, you could make a client--basically, what godwars2 does with it's interface only moved to the end-user's computer (but, you'd have to follow some of the constraints that godwars2 does--there's only a few well-known tiles for dungeons, and most places are completely open; you can't run into trees etc.).
I've seen the telnet protocol, yes, and I agree it sucks. You get no disagreement from me there. Just saying: some of us can't use the new graphical clients that are everywhere. As for supporting both, I've had some discussions about it, and the general consensus is that you need to divorce the interface from the data, which is much easier said than done and probably something you've considered anyway. What would be really nice, ultimately, is probably a new mud communication protocol and clients that support it, but again, much easier said than done (we kind of have this with msdp, you might want to take a look at that).
DGD is really, really nice, though, if you're smart enough to figure it out--I wasn't, but I did try once. I suspect that, more than anything, is why a lot of muds don't use it--as soon as you throw the words "Persistent world" out there, the words "area reset" go away, and the balance headaches start (why have a persistent world if you reset it anyway?).
I'm sure you've probably thought about a lot of this stuff, but you did open it for discussion, so.