The major functionality the new journey planner is missing is the ability to replan around disruption. This was something the old journey planner did very well, indeed I can't think of any other sites that offer something equivalent. Most just state that a given train is delayed or cancelled and leave you to work out what alternatives there are - perhaps fine if just using a direct train, but it's as good as useless once connections become involved. It also means you're not shown alternative routes with changes when direct trains are disrupted.
Furthermore, journeys without fares seem to have the "Journey details" button disabled - so you have to guess where you're supposed to change, if it's not a direct train! When combined with Avanti's fake "reservations compulsory" flag it results in nonsense such as this:
Screenshot showing NRE results for a search from London Euston to Crewe tonight, showing results at 22:00 (cancelled direct train) and 22:03 ('on time' with 1 change but no indication of where the change takes place)
The 22:00 Euston to Crewe has been cancelled tonight, hence reservations have been blocked - resulting in no fares being available. This in turn disables the "journey details" button so you can't see intermediate calling points. Perhaps not the end of the world but it's poorly thought through.
Where this gets worse is when you consider that Avanti have added a Special Stop Order (SSO) on the 22:03 Manchester service for Stafford, allowing a connection onto a WMT Crewe service (this is what the second journey in the screenshot shows). But because this SSO hasn't been reflected in the reservations system, there is 'no availability' for taking the 22:03 from Euston to Stafford, hence no fare can be offered and again the 'journey details' option has been disabled, leaving you guessing where the change is supposed to take place.
It was not at all obvious to me what was going on until I looked at Avanti JourneyCheck together with a list of arrivals from Crewe and joined up the dots. If even an expert like me can be stumped, what chance does the average member of the public have?
Whoever's come up with the specification for this new site needs to have a long, hard think. Sadly I imagine people will be congratulating themselves on "successfully" completing the launch of the new journey planner.