I would say there are a plethora of methods of gaining experience.
- Killin is my bidniss ladies!: Simply killing things probably gets you the least amount of experience. For most of the world there is a 240xp cap on xp from creatures. Factor into that, the xp for creature kills is divided evenly among party members and henchmen/summons/familiars. If you're solo'ing it, you might get some decent xp, but there are better sources.
- Gems: Gems drop nearly everywhere. The meaner the monster, the more likely a more precious gem. Accumulate a few, and take them to Bianco and Son's in Leistra. You will get gold identical to your experience... And between
- Bounty Turn In's: There are several bounties out there. From general bounties like goblin and orc ears, to specific bounties for the Sheriff of Leistra; rewards vary depending on the difficulty to bring down the monster.
- WHAT! Is your QUEST?: Quests are easily the most lucrative source of xp. Rewards are generally party wide (ensure another member of your party has not already turned in your particular quest) and are progressively more and more liberal with their xp. Just quest xp could likely shoot you well beyond lvl 40... then again, you can't really make an omelette without cracking some eggs... so we'd really never know 🙂
While almost all creatures will drop something (pulled from a common loot script), the only way to get exceptional items is to seek out "specific" creatures. We call them "named" creatures. This is where your most epic of loot will stem from.