To me it seems like paladins don't have as wide a range of abilities as other classes. Every other non-dps class has a different set of abilities that it uses while soloing. This makes soloing as interactive as grouping. As well, the game enforces this difference through different means.
Paladins, on the other hand, only use a subset of their abilities while soloing. So soloing is far less interactive than grouping.
Look at warriors. For soloing, they have Battle and Berserker Stances, along with all the associated abilities only used in those stances. Then in group play, they have Defensive Stance and the high threat moves. The different forms enforce the different ability sets.
Druids have different forms (cat, bear, moonkin!) and abilities that they use while soloing. Then in groups, they use a different set of abilities to heal. Like the warrior, the different forms enforce the different ability sets.
Same with Priests. Soloing with a priest is interactive because of abilities like Smite, and various damage spells. Being a priest in a group uses a completely different set of spells. In the priest's case, the different ability sets is more enforced by threat. Mindblast is the perfect example. You don't cast Mindblast in group play because it's high threat. You cast it all the time in solo play.
Shamans have possibly the opposite problem from paladins. In solo play, they can use all their abilities, while in group play they generally use a smaller portion. I believe the game enforces the different ability sets by limiting their mana. A lot of their moves have a high mana cost, making those moves sub-optimal for group play, but perfectly fine while soloing.
A paladin, meanwhile, is pretty interactive in group play. Run in, judge, seal, auto-attack, and then start healing, cleansing, tactical buffing as needed. However, in solo play, it's: run in, judge, seal, auto-attack ... and that's it. The healing, cleansing, tactical buffing that makes paladins somewhat interactive in group play simply doesn't exist in solo play.
The paladin needs a set of moves that it can use in solo play, but CANNOT use in group play.
Unfortunately there doesn't seem to be that much hope for us. We have no forms, unlike the warrior and druid. We can't be limited by threat, unlike priests, because that would make us godlike tanks. And being limited by mana is at cross-purposes with our highly efficient heals.