One thing I'm notcing particularly with the PHB2 classes, the choice to multiclass is very difficult. Abilities are so spread out that switching between classes can really delay useful class abilities. Even with the PHB classes, the decision is a little harder; it was almost expected to multi- or prestige out of Fighter, but now with Weapon Mastery feats, a solo Fighter is much more enticing. Spellcasters have always had this problem, but the Knight has his higher Knight Challenges pushed back, along with Armor use and other abilities.
That said, I'm fond of the idea of a Paladin/Knight- both are heavy armor users, and both can benefit from high charisma. Fighter/Knight is another idea- the extra feats make it easier for the Knight to pick up Shield Specialization and Shield Ward, and still be effective in attacking. I'm running Age of Worms as a Gestalt game, and a Cleric/Knight of Hextor was very effective, but as a multiclass combo you end up losing out on a full Knight's HP and BAB, and a full Cleric's spells. Another combo that just occured to me (because I glanced at someone's suggestion of a Marshall/Knight), Dragon Shaman/Knight: the Knight's martial weapons, heavy armor, BAB and HP, the Dragon Shaman's auras and breaths.