Nope.  That's the problem.  If you find users switching between client
types, you often find trouble calls to go with it because the user will
often pull the mail to a different store (as in POP3) and then wonder why it
isn't there later.  The other protocols do leave it in the central mail
store, so you're right, it wouldn't be a big deal.

Unfortunately, there are no hard and fast rules here.  Every organization
has their own concepts and ideas about storage.  As an example, I've seen
some that believe they can do their job with 10MB limits while others
require 1GB limits.  Others don't even need limits based on the way their
user population utilizes mail and storage.

Exchange can inform and enforce quotas on mail storage.  You can set it for
whatever you want.  I think the best approach is to figure out how long you
can be without mail, figure out how large the stores can become and still be
restored from a total melt-down type of disaster (restored to service vs.
played from tape) and work back to the quota limits from there.  Balance
that with the way folks use email.  I usually go by the 80-10-10 rule.  10%
of the people will use the most space, 80% will use some of it but stay
within whatever limits you set, and 10% when asked will respond "I have
email?"

If you're users are used to sending mail now, you may be able to track the
mail on the system you have and get a feel for the way they do business
today.  That might provide some insight into what you're in for.

Server side rules?  That is so totally dependent and unique based on users
and organization that it's hard to tell.  Many are using the rules today in
their client side activities, but that is only good when that client and
profile are in being used.  If you switch to Exchange, they may be using
different protocols to access the server and may want a consistent look and
feel when they access the mail store.  That would require some server side
rules so that when they use OWA, the rules are already applied.  When they
use MAPI, IMAP, etc the same rules have already been applied and they get
consistency and therefore more productivity (hopefully anyway, right?).

Does that help?

The big one that comes to mind is the out of office notification. I also
use a number of filing rules, moving messages in folders based on
whatever criteria.

That a corporate decision. Bear in mind that usng Exchange with MAPI
client (such as Outlook) enables a host  of options. Group calendaring,
task assignments, etc. If you enable Outlook Web Access (OWA) your users
will be able to not only check for new mail, but fully access all their
mail from any browser, anywhere. Exchange also has retention on deleted
items. You can recover items that were deleted for as long as you decide
(30 days is default). I wouold not recomend using POP3 inside an
organization.