You also operate under a time limit, albeit one that isn't critical. Each stage has a par time, and you gain points for every second under that time but you don't fail for not making it. It's just as well too, because despite a simple premise MMR is by no means a walkover.
The initial stages do a great job of introducing all the different elements, at first individually and then combining them as the complexity begins to ramp up. Colour plays an important role; with many levels having gates and switches that require you to change the colour of your blob to correspond.
This also requires mixing colours, since the painting points only pertain to the primary colours: Blue, Red and Green. To mix colours you must split your blob using the scenery, colour each blob accordingly and then remerge them to get the correct tone to activate the gate, switch and so on. There's a colour code in the top right corner too, so if you weren't paying attention during primary school art then you needn't worry.
Building upon this are the different forms your blob can take. Okay, so a blob is always a blob but its properties can change from normal, hot, cold and solid depending on temperature. When in its hot state the blob runs faster and breaks up more easily, when cold it's slower and more compact and when solid, well, it's a solid. All these states serve a purpose, and provide added challenges based on the design on the level.
These are all interesting, though on their own, basic gameplay elements, and there are a variety of other factors, such as the Mercury devouring creatures and traps, which one must also traverse as you progress. Ultimately, it's the ingenious and constantly challenging level design that brings all these disparate elements together to make this the compelling game that it is.