1. monk can solo push pretty high, but monks' builds for t13 farming are pretty weak now. Mb they are OK for new players, but much weaker compared to wiz and dh.
2. monk is the only healer - it provides active healing with any skill used dew to Epiphany(Smoothing mist), he has 4 passive mantras(read auras) and 1 active mantra(usual choices +10% as / +additional toughness and shields / +resistances and evasion), he has imbalanced Inner Sanctuary(15 AoE area with 55% dmg reduction for all party) and he also can reduce dmg of the mobs and buff dmg or his allies a bit.
3. in solo you can choose there are also range and melee combat builds. But as healer - monk is the best tank-support-healer
4. meta changes from patch to patch, 95% chance that support-barb and support-monk will be in meta in 2.6. Two other roles - we don't know, just for now it is WD as mob killer, Dh/Wiz as boss killer. But Necro can take any dmg role - we don't know how powerfull he is now)
5. wizard is the best for solo push, for keyfarming and for low lvl speed 80-90gr - where most people farm XP and loot. This setup is 3 dd wiz + 1 monk heal. Also - necro can take this speedfarm role too) will see.