While not necessarily easy to find, 7.4 features have been posted on their feature comparison page for some time now. 

http://www.e-onsoftware.com/products/vue/vue_7_infinite/index.php?page=14&BaseProduct=Vue%207%20Infinite

It's getting more and more clear, the Infinite line is for professionals who can afford the subscription plan. To this end, they have made  pretty decent offer:

Pay now for the maintenance plan, and get immediate access to the 7.5 beta (by all reports suprisingly stable), then when 7.5 ships you also receive it. And then when 8.0 ships, you also get it as well. All for $395. Not too bad. And if you don't want any of this, then they will be rolling many of the 7.5 features into 7.4, which is a free upgrade to 7.0.

I have the beta 7.5, and it's not a rewrite of the whole application. Furthermore, I seriously doubt they would rewrite the whole application for 8.0, especially when they are talking about it being a free upgrade for those who purchase maintenance plans.