Funny interstar should mention Redis. I've actually reverse engineered almost all of Redis's behaviour using Mumps (both Cache and GT.M). Emulating its simplistic and basic key/value pair structure and APIs has been a piece of cake and the Redis clients I've tested don't even know they're talking to a Mumps database instead of the proper Redis.
However, like interstar says, the Mumps database is capable of so much more in terms of both its data structure (actually a Mumps database can easily emulate all 4 NoSQL database types: key/value, tabular, document and graph, and can also model relational and object dbs - all at once if needed), and the properly sorted out and automatic balance between in-memory versus on disk. Unlike the NoSQL databases that have been around for just a few years, the Mumps technologies are mature, tried, tested, well supported and have analytics and administration packages available, and have the kind of performance that many NoSQL databases such as CouchDB would die for.
So why aren't the vendors of Cache and GT.M promoting hard into the NoSQL market rather than let someone reinvent their wheel as you say?
As to the tired old bickering about the language and the use of that ancient and outdatad example from the Wikipedia site:
a) The Mumps database can be accessed by your pet sexy modern language of choice if you want
b) that style of programming went out with the ark, and was written at the time like that to squeeze amazing performance from PDP computers that had less processing power than the chip in your watch. Modern Mumps coding is actually very similar in style to Python and can be just as clean, readable and maintainable.
c) Like any language there are good examples you can find and terrible ones. Don't assume that Mumps coding needs to be as opaque and old-fashioned as the example on the Wikipedia site.
..and Mason, my conclusion would be that perhaps you aren't as familiar with the database landscape as you thought you were! :-) If you've had any form of medical treatment in your life, your information will have been through umpteen Mumps systems along the way, including ones that manage requesting of your tests. The contents of your daily newspaper may be managed by a Mumps system and even the pizza you ordered online was probably via a Mumps system too. The logistics of items you might have had shipped overseas was probably managed via a Mumps system, and if you happened to be a researcher who needed some fancy metals or ceramics, you'll have probably ordered them via a Mumps system.
One of the reasons you rarely hear about what is actually a surprisingly pervasive database is that the vendors have traditionally dealt directly with application/system developers rather than end users, so the database is usually embedded behind the scenes into products.