Soloing isn't bad - my ratonga cleric has very little difficulty soloing things at or near level and has completed a lot of quests (107 or something) without group assistance.
That said, there are things that are a lot easier to do as a group, especially some quests. If you have to kill 20 orcs, and each of your friends has to kill 20 orcs, it's a lot easier to kill orcs together because group kills count for each of you. So, instead of needing to kill 120 orcs for fighting seperately, a group can kill 20 orcs, total, and all get the credit needed to complete their quests.
Solo play relies heavily on quest stacking to achieve decent experience gain - find quests that call for the same victory conditions, so you get multiple quest credits for completing a single set of requirements.
Still, if you solo, there are some things you're going to have a hard time with. Some zones are heavily stilted towards grouping, because they have large camps of mobs that you can't take on by yourself. There are a few upper-end quests that you also can't solo.
Basically, there's plenty to do on your own, but there is more you can do in a group.
Sam