Every class has its weakness. As a druid, my very strength (diversity and adaptability) is my weakness, because specialising in one area will detriment me in another. For example, as a balance druid I have a lot of item bonuses invested in intellect and spirit - giving me a massive mana pool and very fast regen rate. However, this means my stamina, strength and agility are very low - not great for soloing or melée. This is made worse by the fact that I'm a PvP player, so power and survivability are essental.
Mages have the highest spellcasting damage in the game, regardless of your experience. Their spells by definition focus on raw power - but they're piss weak when you hit them with something. Warlocks focus more on DoTs - damage over time spells. Furthermore, warlocks are like hunters in their dependency upon a pet - you have less crowd control measures at your disposal, and can be virtually crippled if your pet is incapacitated (yes even with chain fears).
Private servers are definitely not worth playing an MMO type game on. The whole experience is based upon the concept of being surrounded by thousands of other player-characters at any given time, impossible on a private server. You will get very lonely, and very bored, very fast.
When I started my monster 60 run I was one rested bubble or less from 59. It honestly shouldn't have taken nearly that long, but in one of the DM runs I was helping a mage get her awesome [Conjured Crystal Water] (best mana restorative in the game) and, well, she was shit. Didn't know how to DPS, kept pulling aggro off the tank and getting us wiped. I really should have left but as the only healer available to them I felt much pity - especially since I myself depend on mages giving me their funky mana juice.