I'm a longtime Smartlist admin who's migrating to Mailman 2.0.6 and I've got a few questions regarding features:

I have several legitimate 'non-opt-out' type of lists; Ie: our department wants to make sure that *every* faculty member is given a certain message, so we use a smartlist mailing list. I've configured it so that users can not unsubscribe from that list - they have to contact us directly [and we only unsub someone if they are quitting]. Is there a way to do this in Mailman?

Not until 2.1, but yes.

Also, seeing as someone may want to get around that by putting themselves on permanent vacation, is there any way to disable that feature as well?

Not without hacking, but it's probably a small amount of hacking. A reasonable RFE.

Is there any way to have a reduced-feature admin w3 page? Ie: I'd like to give a non-tech person the ability to add/delete users, but nothing more. I don't want them to change the site URL, name of the list, etc, etc.

Again, not until 2.1, but 2.1 has the concept of "moderators"; here's the description of what they can do:

The list moderators have more limited permissions; they are not able to change any list configuration variable, but they are allowed to tend to pending administration requests, including approving or rejecting held subscription requests, and disposing of held postings. Of course, the list administrators can also tend to pending requests.

I think this does not, at present, allow direct subscription by the moderator, but since the moderator can format up an email message and send it, and then approve it, effectively he can do one-off requests. Adding the ability to change the subscriber list only would be a reasonable RFE. Looks like it currently grants the ability to deal with admin requests, to see archives, and to see the membership roster.

Is there a way to either globally or on a per-list basis, customize some of the auto-reply text? I notice you can prepend some text to some of the auto-replies, but how 'bout replacing the entire auto-reply message itself?

Again, in 2.1 it's easier; the list messages are internationalized, and as part of that process they can now be made list-specific with a specifically-defined search order:

# When looking for a template in a specific language, there are 4 places
# that are searched, in this order:
#
# 1. the list-specific language directory
#    lists/<listname>/<language>
#
# 2. the domain-specific language directory
#    templates/<list.host_name>/<language>
#
# 3. the site-wide language directory
#    templates/site/<language>
#
# 4. the global default language directory
#    templates/<language>