What could happen if each of the four fundamental forces became stronger or weaker? Theoretically, what could happen if:


*

*Strong interaction

*Weak interaction

*Gravitation

*Electromagnetism


Became stronger or weaker?
What would be the observable effects for each, separately?
 A: Just to start (I'll keep updating this as I find more):


*

*If the strong nuclear force (the force that binds quarks together into protons and neutrons and binds protons and neutrons together in the nucleus) was stronger or weaker by 5%, life would be impossible.

*If gravity was stronger, stars would burn out faster; this is true to the point that if gravity was stronger by a factor of 10, the sun would have already reached red giant size and Earth would no longer exist. 

*If the nuclear weak force was appreciably stronger, the Big Bang would have burned all hydrogen to helium, resulting in the lack of both water and stable stars. If it was appreciably weaker, none of the neutrons at the Big Bang could've decayed into protons. 

*If the strong nuclear force was 1% stronger or weaker than its current value, carbon could not be created in quantity inside stars, and if it was 2% stronger, either the formation of protons would be blocked, so there wouldn't be any atoms, or bind them into diprotons, making stellar burning impossible.

*If the electromagnetic force was slightly stronger, main sequence stars would be too cold to encourage life. If it was slightly weaker, main sequence stars would be to hot to encourage life, and they would also be very short-lived. A slight strengthening could also transform all quarks into leptons or else make protons repel each other strongly enough that even the lightest atoms couldn't exist.


A book with more information is John Leslie's book Universes; here is a link to the google book.
Hope this helps!
A: The strong force
In our universe the strong nuclear force is not quite strong enough to hold the helium-2 nucleus together, so helium-2 cannot exist. But if the strong force were just a tiny bit stronger then helium-2 would exist and the fusion of hydrogen into helium-2 would be possible, and the world would be different. 
How much stronger? 


*

*Barrow and Tipler say 2.4%. 

*MacDonald and Mullan  quote a paper of Barrow as saying 13%.

*Wikipedia quotes R.A.W. Bradford as saying 2%, although in fact Bradford’s paper gives no figure at all.


But the whole argument is largely nonsense anyway. R.A.W. Bradford, in The Effect of Hypothetical Diproton Stability on the Universe (J. Astrophys. Astr. (2009) 30, 119–131), has calculated the early cosmos for various strengths of the strong force, up to 40% stronger than our own. His calculations show that the amount of helium-2 produced in the Big Bang is… none at all. It is not true that, as Barrow and Tipler say, all the hydrogen in the universe is turned into helium-2. On the contrary, none of it is.
It is not that the fusion reaction doesn’t exist: it does. But it is slow, and the Big Bang cools down fast, so that it is too cold for fusion long before a measurable amount of helium-2 has been made. Moreover, in the first second of the Big Bang, when the universe is hot enough for a proton-proton reaction, there is so much heat and light around that it instantly frazzles any diprotons that may have formed and blasts them to bits.

As for the tightness of the strong force constraint, in the arXiv paper Big Bang Nucleosynthesis: The Strong Nuclear Force meets the Weak Anthropic Principle, J. MacDonald and D.J. Mullan present detailed calculations showing how the synthesis of various nuclei in the Big Bang would be affected by a strong force 10%, 20%, 30%,… stronger than the one we have. Their conclusion is ‘Our main result is that the existence of bound diproton and dineutron nuclei does not necessarily lead to complete conversion of hydrogen to helium in the big bang. Instead there are parameter ranges for which significant amounts of hydrogen remain… [T]he final hydrogen abundance is greater than 50% of the standard BBN [Big Bang nucleosynthesis] value for increases in the strong force coupling constant less than about 50%. Anthropic limits on the strong force strength from BBN are indeed weak. ‘
The Hoyle resonance
When beryllium-8 collides with helium-4 inside the right sort of star, carbon-12 is made. There is a great deal of surplus energy, which means that the carbon-12 nucleus is in a highly excited state and blows itself to bits before it can get its act together enough to create a particle pair and emit a gamma ray, emit the surplus energy and settle down into a calm and respectable middle age. As a result there is no carbon-12, and we don't exist either.
What saves the world and makes us exist is that there is a resonance of the carbon-12 nucleus, at $7.656$MeV, which means that the excited nucleus rings like a bell for long enough for the surplus energy to be omitted. Which means that carbon-12 does exist, and so do we.
Now, people sometimes think that this means that the strong force mustn't be other than it is (to high precision) otherwise the energy generated by the fusion would be wrong and the Hoyle resonance wouldn't be able to rescue us. This is nonsense.


*

*The dependence of the energy of fusion on the strength of the strong force is calculable. The dependence on the energy of the Hoyle resonance on the strength of the strong force (strictly speaking, the strong forces between quarks) is beyond our ability to calculate at present. It was hard enough to calculate the resonance based on the forces as they are now: A calculation was finally made, using supercomputers, in 2011, and refined in late 2012, but even then it could only be done with some simplifying approximations. Evgeny Epelbaum, Hermann Krebs, Timo A. Lähde, Dean Lee and Ulf-G. Meißner, ‘Structure and Rotations of the Hoyle State’, Phys. Rev. Lett. 109, 252501 (2012). It is entirely conceivable that the energy of the Hoyle resonance is dependent on the strong force in exactly the same way as the energy of the fusion reaction is, so that whatever the strength of the strong force, the Hoyle resonance ensures that the carbon-12 survives.

*If the Hoyle resonance is slightly out of tune, all sorts of reactions happen differently, not just the carbon one. The net result is that the star makes less carbon but is also less stable, which means that it is more inclined to spit plumes of gas into the interstellar medium. Less carbon is made but we get to see more of it. So there is plenty of carbon for us to be made out of. M. Livio, D. Hollowell, A. Weiss, J.W. Truran, ‘The anthropic significance of the existence of an excited state of $^{12}$C’, Nature 340, 281-284 (1989).
