Since new sfdc customers cant use HVCP anymore, I want to do a similar app to ones I have built with portal, what are the key difference here? Anything I cant do anymore? Anything new I can do?
I'm not an expert on communities, but I can compare high volume customer portal with the now-deprecated customer portal license.
High volume customer portal users are something like 25% the cost, per user, of customer portal users. In exchange for this they aren't able to take advantage of sharing. Instead they're governed by much simpler permission grants that are defined on the customer portal setup page. Since you're using HVCP today I don't think I need to elaborate too much on that feature.
Instead of that customer portal (and I'm pretty sure communities) users get to work with normal sharing, that is they follow the org-wide defaults (although note that there's now separate defaults for internal and external users) and sharing rules, be they apex, criteria, or hierarchical, do impact their ability to view/edit records.
I believe there's also some system permissions that aren't available to high volume customer portal users, but off the top of my head I don't know what they are. The sharing settings were by far the most key difference between the two types of licenses.