Lordaeron's core is a revised and thoroughly updated iteration of Trinity Core. It's vastly different from the old Trinity Core framework upon which Ragnaros/Deathwing were based and developed. The general quality of Lordaeron is in a different league from that of Ragnaros and Deathwing. They really don't even begin to be comparable; most players will probably feel the difference immediately. Mob LoS is non-existent on Deathwing and path-finding is downright dismal - these are close to flawless on Lordaeron. The state of questing is likewise simply superb by contrast. A lot of the critical class bugs that significantly alter what classes you'd bring to a raid, like wolf pets serving as big-time melee DPS buffers, Demo Lock and Ele Shaman spellpower buffs stacking, etc, are largely all completely fixed, as are several other class bugs that greatly affect gameplay. It's not perfect for the time being, but for the most part it's a massive cut above what we have on Ragnaros/Deathwing right now.
Every Northrend dungeon actually works (and fantastically, I might add). Right now when you queue into an RDF on Deathwing, there's a good chance you're gonna run into a bugged instance (Occulus, UP, ToC), and end up having to re-make the group to get your dungeon rewards at the end.