Weighing in with the other side of Tinpony/Linneth's post, as a Mage/Harvester it boggles the mind why anyone would stay on n00b Island past the low 20s.
At 23 Magic/20ish Harvest (honestly forget the exact harvest level
) the XP was horrid, the quality of mats available was capped five lower than my skill, and there were irritating Raguses everywhere. Not really a threat, but annoying in multiples and a pain to kill while you're kiting.
Once arriving on the mainland options improve significantly. There is a high learning curve at first, fortunately I was able to get that over with during the beta. Once you know the approximate levels of what you're facing (they really need to fix the con display), where it's safe to hunt, and have a full understanding of social aggro the mainland is the place to be. Better rewards and better XP are a winner IMHO.
For a mage your bread and butter for XPing will likely be fear kiting, in the newbie lands this tapers off pretty quick. The XP for a Puny Ragus or such is pretty poor once you hit 20, and Kipee require investing in the cold nukes line. (not recommended that early on) On the mainland you have a wide level-range of Yubos to hunt, they don't have social aggro with anything, they don't really hit particularly hard, and at later levels (Scampering in the 30s and Sprightly in the 40s) they drop very good staff components.
As a harvester the mainland opens up a wide variety of options in terms of materials including choice mats and mats not available on n00b island. One poster commented on towns being primarily amber, while this is true in terms of pure XP grinding there are also far more sources available in town, and when XP grinding the type of mat is rarely relevent as it's mostly vendor fodder. A short distance outside of town will provide plenty of variety in materials. (in Zora there are different types of mats harvestable just barely outside of the wisps at the enterence)
N00b Island is useful to get the first few levels under your belt I suppose, although suckling yubo and mektoub are available in large quantities on the mainland for low leveling purposes. Staying there beyond level 20 or so seems like a mistake.