There are a surprising number of legal and regulatory requirements for everything from footer links proper coding. At the enterprise level, we maintain all that for you, so you don't have to think about. 

From my own perspective, the most important reason to migrate to AFPIMS 5 is legal compliance. The previous software was built on a grid, similar to an Excel spreadsheet (called tables in HTML). This methodology was a common HTML architecture for websites a decade or two ago. However, it is now considered a violation of the Rehabilitation Act, Section 508. Tables can still be used to display data, as you would information in an Excel spreadsheet, but not to organize photos, videos and standard content. The reason is because the standard text to speech browser reads table data from left to right, as you would normally read an Excel spreadsheet. However, tables were normally used to organize websites vertically, by putting a photo above a cutline which was above a headline and all of this above the byline and story. Sighted users would read this content from top to bottom.
However, non-sighted users browsers can't determine the most appropriate sequence to read the content.

Most web hosting structures promise the website will be up 99% of the time. Two Nines equals to 7 hours and 12 minutes downtime in 30 days.  Since I began working at AFPAA in May 2011, our sites have never been offline, because all the sites are mirrored on Akamai, the world's largest cloud computing company. ePubs was offline for 3 months on the backend, but the world never knew it because Akamai went on serving those pages and people kept on reading them.  When the non-AFPIMS USFK website went down for a little more than a day, they made newspaper headlines. We aren't an average business and an average business solution isn't sufficient for us. 

Two years ago, I located 30+ non-AFPIMS official USAF websites and informed their owners of the security vulnerabilities those sites presented. Last year, two of those sites were hacked and defaced by ISIS sympathizers. Again, the Department of Defense isn't an average business and an average business solution isn't sufficient for us. Everybody in the world wants to hack a DOD website. We need a significantly robust security system. 
From a user's perspective, AFPIMS 2 was a nightmare when it came to loading videos. Users repeatedly complained that a video upload would often time out several times and could take all day to get one video loaded. Among other things, the new structure has separated video so that now all videos in AFPIMS 5 come from a secondary source of your choosing, which is either DVIDS or YouTube. Once you make a channel on either of these sites and link it to your AFPIMS website, videos upload to that location are visible in the backend and can be selected and placed on the website. Users will be unaware the video is pulling from a different server/source. Users are particularly fond of DVIDS because it automatically assessions their videos to DIMOC and, if they connect their DVIDS channel to the unit's YouTube channel, it will automatically populate that as well. Moreover, DVIDS has an automatic closed captioning service, another element of Section 508 compliance. 

As alluded to in the video segment, social media including YouTube has been deeply integrated into AFPIMS 5. This was non-existent in AFPIMS 2. In addition to the video integration with YouTube, we have a new live feed module that integrates both Facebook and Twitter. Integrating social media was one of my top priorities. This includes a new website description field in the metadata, which will auto-populate whenever someone pastes the URL into a Facebook post. AFPIMS 2 didn't populate anything. 

One of the top user complaints regarding AFPIMS websites has always been search. To optimize search, we need metadata. AFPIMS 2 didn't have much in the way of metadata. AFPIMS 5 has tags, categories, website and page descriptions and a host of items the webmaster can fill in to improve the search results for their website. That said, most of our PA Career Field members don't understand metadata or SEO (Search Engine Optimization) and/or the proper use of tags, categories and descriptions. We'll need to improve our education, but the technology is now aligned with best practices for the Internet. 

APFIMS 2 was considerably more difficult to update because the backend and the frontend displayed differently. A backend page on AFPIMS 5.0 looks pretty much the same as what the user sees except that each module has an edit and settings button which the webmaster can use to update that portion.
This is considerably easier than the old system.

In AFPIMS 2, if a member had edit access, they automatically had edit access to all the pages in a website. Now, we can grant a single user rights to a single page, several pages, or even a single module. This gives PAs an excellent option to offload some work requirements. They could give their JAG, Chaplain, MWR, HR, MTF, Dental facility and other appropriate POCs on the installation rights to their content. 

AFPIMS 2 was a proprietary system. This means, it could only have as many modules and functions as a couple of programmers could produce. AFPIMS 5.0 is built on DotNetNuke, an open source system. This means anyone in the world who wants to create a module for the software can and they can submit it to a central change control board and if accepted it would added to the overall functions. Of course, this creates security concerns, so not all modules are immediately available. The DMA security team must vet each module before it's approved and added to our instance. However, there are literally hundreds of options available. Some require payment for licensing.
However, the range of capabilities is exponentially greater than AFPIMS 2. 

One element customers, specifically Air University, have commented on is the sharp visual presentation of content and the ability to customize the background image for branding.
Here are a few examples: