It may be a lack of rigidity, poor straight line performance, or poor secondary stability. A keel is also a major liability if you run whitewater. Where a canoe without a keel will slid over a rock, a canoe with a keel may hang on the rock and give you an unexpected swim. If you plan to do any moving water, you should stay away from an external keel. Canoes that do not have a keel, or only have an internal one in the case of aluminum canoes are typically of a higher quality.
Rocker is how much the bow and stern of the canoe curves upward.
This should not be confused with stem as a canoe can have a square stem, but still have a lot of rocker if the keel line extends all the way to the bow, or even further. Like stem, a canoe with a lot of rocker is easier to steer, but sacrifices straight line performance. If you plan to do whitewater canoeing in extreme conditions, you will probably want a lot of rocker, more than three inches is not uncommon. A canoe under load will sit deeper in the water, and if it lacks rocker, the bow of the canoe digging into the water will make it difficult to steer. If you plan to do some light cruising, then only a small amount of rocker is required.