Each version has Release Notes. For the ASA they are all posted here.
In each Release Note there is a "Resolved Caveats" sections. That is where the fixes for all problems - vulnerabilities as well as functions/features - are listed.
Besides higher encryption and Anyconnect client, you can also use IKE v2 (as of 8.4(1) ) which is more secure during session setup (apart from the level of encryption). You can also use identity-based features and a host of other features to further secure your remote access VPNs. On the other hand, if what you have now is meeting your needs, the only compelling reasons to upgrade are vulnerability and bug fixes (and perhaps a prettier version of ASDM that will run with the newest Java versions ).
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...