There is nothing wrong with playing solo until level 60.
The only dilemma is when you reach the end game your options are pretty much limited. You can still do quests and instances but without the xp, it becomes a farming session (gold/gear). That can get pretty old quick.
I practically solo'd all the way to 60. I did some instances but mostly leveled thru quests and grinding. But once I hit 60, what else can you do? Raid or PVP. Those were the options. I didn't feel like rolling an alt and go thru the growing pains again. Rolling on a different server didn't sound exciting either. So I took the solo way to the end-game: PVP.