While you're adding features to DND, I'd like a timed DND. CallVantage offers a scheduled DND (though their scheduling options are rather limited), which is nice; they do not offer a permanent-on DND. Their "do not disturb now" feature is timed in 30 min increments for up to 3 hours, I think. Once time is up, DND is canceled without having to dial a feature code or log in to the web portal. (You can extend DND by as much as you want, so it can be set to auto-timeout after well over three hours.)
The other nice feature of CallVantage DND is that the inbound call is intercepted by a recording (can make a custom one) in which the caller is given the option to 'ring through' (such as for an urgent matter). Otherwise, they are transferred to voicemail to leave a message. I realize that with that function, you're trusting the sleazy telemarketers to not wake your 6 month old up during nap, but it's nice to have, IMO. (Of course, since not everyone would like it, you could add an 'Enable Urgent Call Completion' option for each user to decide whether they want to allow callers to override their DND. If you're going to make the feature more complicated, you might as well go all the way! )