Immediately this took my mind to DC's Dr. Manhattan, or maybe also the main character from "The World Walker". What both these characters have in common is a struggle to retain their humanity - just a thought.
Honestly, this power could manifest in thousands of different ways, but it really depends how biologically accurate you want to be... If a person were to have complete control at a sub-cellular level, then some possibilities are:
1. Immunity to all infectious, genetic, inflammatory, and auto-immune diseases, and cancer (infectious diseases could mutate to still cause infection)
2. Increasing recovery speeds for wounds, and not suffering from inflammation (still limited by the movement speeds of cells, proteins and other molecules)
3. Improved reaction times (still limited by the speeds of action potentials and synaptic transfer)
4. Perfect memory due to perfect formation, organization and maintainance of neural pathways
5. Complete sensory control - could turn off or on any sense at will, or make them more sensitive by increasing density of sensory neurons or accessory cell types.
6. Inhibition or manifestation of hormonal/neural signals that result in any sensation from pain to hunger to happiness
7. Perfect control of muscular contraction, hypertrophy and growth
8. At will gene editing in any part of the body, to produce any protein known to man to perform literally countless functions found in any biological organism or system known to man
9. Control over pigmentation of any body part
10. Grow or reduce in size, or add extra parts - forced mitosis and differentiation
11. Become completely amorphous - forced de-differentiation and transformation
12. Control over general metabolism to be able to enter death-like states and stasis
Almost all of these things would likely be limited by the physics surrounding particle collision and cellular metabolism; would require insane levels of food/fuel and cofactor intake to provide the necessary energy; and would require extra methods of heat dispersal (otherwise the person would need to sweat a lot all the time) and free-radical soaking, among a bunch of other side effects that I can't think of off the top of my head.
You could go more general than this however, and focus on the general function of specific cell types:
1. Muscle cells - contract to provide motive force
2. B Lymphocytes - produce antibodies generally against foreign material
3. Rod and Cone cells - produce photopigments to convert light into molecular signals
4. Neurones - propogate action potentials
5. Pluripotent cells - differentiate into many types of cells
etc.
I think I've generally developed a slight aversion to powers/magic systems that are as general as this when applied to only one character, especially if there are no trade-offs. That said, I basically gave my MR Canon character this kind of power in a much more limited sense, whereby he can effect a small selection of physiological attributes by listening to specific styles of music. But with intense or prolonged use, this results in him suffering related side effects or could even lead to his death if he pushed his body too far.
Personally, I would probably break all these possibilities for cellular control out across several characters, whereby they each control a single cellular function in a specific way.
Hope this is all slightly useful.