Over the last few months, Dave Reid, one of the most active Drupal developers, has been giving a presentation called "Future-proof your Drupal 7 site".
Dave talks about decisions you can make now on existing or new Drupal 7 sites to make transitioning to Drupal 8 easier. He comes up with a list of modules that have been backported to Drupal 8. Using those modules means you won't have to re-train your staff for Drupal 8.
Dave also has some recommendation for modules not to use, because they've been replaced by alternative solutions in Drupal 8. Here are 6 modules that may be worth avoiding if want an easier update to Drupal 8 in years to come ...
#1. The Node reference or User reference modules
Use the Entity reference module instead: https://www.drupal.org/project/entityreference.
#2. The Phone module
Use the Telephone module instead: https://www.drupal.org/project/telephone.
#3. The Link module
Use the URL module instead: https://www.drupal.org/project/url.
#4. The Services module
Use the RESTful Web Services module instead: https://www.drupal.org/project/restws.
#5. The Date module (some formats)
The Date module is still useful, but make sure to use the Date (ISO format) field type, not the Date or Date (Unix timestamp) field types.
#6. Display Suite
The advice from Dave's presentation is Austin was not that Display Suite is on the way out. In fact, there's already an Alpha release of Display Suite for Drupal 8.
Instead, Dave's advice was that if you use Entity View Modes you'll stick close to the Drupal 8 core and possibly have an easier update.
10 modules in Drupal 7 won't move to Drupal 8
There were 10 core modules removed from Drupal 7. Several were merged into other Drupal 8 core modules and several have become stand-alone modules on Drupal.org. This page has specific details of each module's fate.
- PHP filter
- Poll module
Watch the full "Future-proof your Drupal 7 site" video
To get a fuller explanation of all of this, it's worth watching the video of Dave's talk. The are several recordings online, but here's the latest version from DrupalCon Amsterdam: