I work as a Systems Engineer/Developer for an ISP, so unfortunately I've been exposed to the difficulties of configuring a decent antispam engine (our use case is different to that of a personal MX however, and some of the techniques we have to apply to scale wouldn't apply). I'd consider a decent antispam setup one which:
* performed a full validation at SMTP time of the various protocol bits and bobs (ie. the sending SMTP follows the RFCs)
* respects DKIM and SPF
* performs an AV check (clamav is fine here)
* greylists
* performs a bayesian analysis
* uses RBLs like DNSBL and the like
* supported encryption/TLS for SMTP
Overall, setting something like that up isn't
so difficult, but it is time consuming and fiddly. A turnkey virtual appliance which configured Postfix, Dovecot and all the other extensions would be very useful. FWIW, after posting this yesterday I found
http://www.iredmail.org/ which actually does all that - so fired it up on a Linode and was very impressed :).
I believe there's a market for it because I believe there's a growing amount of users who want to switch off Gmail (and other cloud email providers), considering posts like this. The threat of losing your communications infrastructure is pretty terrifying for most; I funnel mail from 3 or 4 different domains to my Gmail account and if it got taken away I'd be screwed. As people become distrustful of cloud providers - where else would they go? A simple "run your own email" appliance would give those concerned with their privacy and security online the ability to setup their own instance without having to delve into the arcane magic that is configuring an MX in this day and age :).