What Actually Changed (and What Didn't)
Backwards-compatible at the adventure level. Different enough at the character level that you need to pick one.
The 2024 rules are backwards-compatible with 2014 adventures and monsters — you can run Curse of Strahd with 2024 characters without conversion work. But character creation and progression changed substantially: every class got revisions (some minor like Rogue, some major like Paladin), weapon mastery gives martial characters new options every attack, background determines starting ability scores, and feats are now baseline (every character gets an origin feat at level 1).
The community's consensus: "compatible" means adventures and sourcebooks still work, not that you can mix 2014 and 2024 characters at the same table without friction. A 2024 Paladin next to a 2014 Fighter will create a visible power and options gap. Pick one ruleset for the whole table per campaign.
Start Fresh or Stay Put: The Decision Matrix
New campaign with new characters: go 2024. Mid-campaign with existing 2014 characters: stay. Mixed group: go 2014 for simplicity.
Start fresh with 2024 if: you're beginning a new campaign, your players are willing to learn updated classes, and your table enjoys tactical variety (weapon mastery adds a decision point to every martial attack). Stay with 2014 if: you're mid-campaign with characters people love, your table has players who don't want to relearn their class, or you have a significant investment in 2014 books and don't want to buy replacements.
The community's practical compromise: use 2024 rules for new campaigns, keep 2014 for ongoing ones, and borrow individual 2024 rules (weapon mastery, the new exhaustion system) as house rules in 2014 games. This hybrid approach is the most common pattern in actual play discussions.