Rewards come in many forms. For the characters, it comes in the form of treasure and power, gold and magic items. The awarding of such treasures is always a balancing act for a dungeon master. Too little, and the characters will suffer and struggle. D&D, and most other RPGs are based around character advancement. If you don't give the PCs what they need to advance... then the game doesn't work the way it's supposed to. Too much and you end up with a Monty Haul game.
Unfortunately there aren't many hard and fast rules about what's too much or too little treasure.
Finding that balance, which is different not only for every game, but also for every campaign, takes practice. Games like 4e are pretty clear about what it expects the balance to be, and it's built around that pretty clearly. Even so, there is still room for individual preference. I know that some groups gain a level every time they play, even if based on what actually happened it should be 3 or 4 sessions between gaining levels.
Classic D&D is a lot more flexible, and as such requires a little more finesse from the DM. My strategy is to be a little on the generous side with rewards, but also to make the players either earn it, or work to keep it. And that's one of the best techniques a DM can use when they've given out too much. Gold can be stolen while the characters are out adventuring. 2nd level characters with holy avengers are going to be hunted down by those who think they're more worthy to have it. They might not kill the PCs, but they'll do their best to take it from them once word gets out. I don't recommend this be used often. It will piss of the player unless you really do it right. It can't be pure DM fiat, it has to work in the context of the game. Pull it off, and it'll be something your players will remember for a long long time.
This post is number 18 in the 30 Days of Gamemastering Challenge.