When I boil a kettle, what stops all the water from turning (exploding!) in to steam in one go once it reaches 100°C?
Energy is needed to convert water to steam. This is called the latent heat of vapourisation and for water it is 2.26 MJ/kg. So to boil away 1 kg (about a litre) of water at 100 °C the kettle would need to supply 2.26 MJ. Assuming the kettle has a power of 1 kW this would take 2260 seconds.
Given the unexpected interest in this question let me expand a bit on what happens to the water. Suppose we start with water at room temperature and we turn the kettle on. We'll take the power of the element to be $W$ (units of joules per second) so we have $W$ J/s going into the water. This power can be used for two purposes:
to heat the water
to evaporate (boil away) the water
Let the rate of temperature increase per second be $\Delta T$, then the power used for this increase is $C\,\Delta T$, where $C$ is the specific heat of the water. Let the rate of evaporation be $\Delta M$ kg/s, then the power used to evaporate the water is $L\,\Delta M$, where $L$ is the latent heat of vapourisation. These two must add up to the power being supplied so:
$$ W = C\,\Delta T + L\,\Delta M $$
When we start heating, and the water is cool, the rate of evaporation is very low so we can ignore it and say $\Delta M \approx 0$. In that case we find the water heats up at a rate of:
$$ \Delta T = \frac{W}{C} $$
When the water is boiling the rate of temperature increase is zero because the water can't get (much) hotter than 100 °C so $\Delta T = 0$. In that case we find the water evaporates at a rate of:
$$ \Delta M = \frac{W}{L} $$
So at the start the water is mainly getting hotter at a rate of $W/C$ degrees per second, and when boiling the water is turning to steam at a rate of $W/L$ kilograms per second. In between the water will be both getting hotter and evaporating at some rate lower than these two limits.
Since neither of the answers given so far really answers the question, here's my 2 cents' worth :
Between convection (the flow of water of various temperatures around the kettle), and the fact that the heating element is at the bottom, the water is at various temperatures at various parts of the kettle at any time. Usually, the hottest is at the bottom, if the kettle is on top of a heating element. Indeed, you can see with a glass pot of water that the bubbles in fact form at (or very close to) the bottom.
Also, there's a factor that's not so important for kettles, but otherwise can be relevant: nucleation points. When a phase change (like liquid to gas) occurs, it normally begins at a location that has some sort of disturbance, maybe a speck of impurity in the water, a locally significant temperature fluctuation, a slight imperfection in the surface of the (inside of the) bottom of the kettle, that sort of thing. That's why, even though lowering the temperature increases the solubility of CO2 in H2O, dropping an ice cube into soda releases a lot of gas: the abundance of nucleation points on the surface of the ice allows the dissolved CO2 to un-dissolve and form bubbles.
So, heating a mug of water in a microwave where the volume of water is small enough to be uniformly heated, and it's heated from the inside rather than the outside, it is possible for the whole mug to get to 100°C at the same time. If the inside of the mug is really smooth and the water's really pure, it could even super-heat to a bit above 100°C. At this point, a slight disturbance (like putting in a spoon or some sugar, or moving it and making waves) can create a nucleation point and lead to much of the mug boiling at the same time. This is both very cool and quite dangerous: if you go to a big city ER you will find they treat people with badly burned faces due to this effect, once in a while.
Temperature is a measure of average kinetic energy. When you have a kettle of water at 100˚C, some of the water molecules will have more-than-average energy, and some will have less. The more-than-average molecules are the ones that will turn to steam, carrying off their energy and lowering the average (and thus the temperature) for the remaining water.
This is why you start seeing steam before the kettle reaches 100˚C, why you need to keep adding heat after the kettle reaches that temperature, and why the water in the kettle doesn't all boil at once.