Omnipotence, sometimes known as being all-powerful, refers to God's ability to do absolutely anything God wants. This characteristic is usually treated as implied from God's characteristic as absolute creator. If God is capable of creating all of existence (whether ex nihilo or ex deo), it is felt that it would be nonsensical to then assert that there are things beyond God's abilities. Any being capable of creating existence itself must therefore be capable of anything at all - right?

Unfortunately, the most absolute sense of omnipotent has been found to be incoherent. If God were truly omnipotent in an absolute and unlimited sense, then God could be capable of both existing and not existing at the same time, meaning that every form of theism and every form of atheism would be equally justified at all times simultaneously. Such a God could be capable of informing humans of certain requirements for attaining heaven and avoiding hell but actually holding to entirely different requirements without ever actually lying.
Clearly, then, any coherent understanding of God and God's nature requires theologians to place limits of some sort upon God's alleged omnipotence. The first and most basic limit, designed to avoid problems like those described above, is that of logic: God's omnipotence means that God can do anything that is logically possible to do. Thus, God cannot make 2 + 2 equal 5, God cannot both simultaneously exist and not exist, and God cannot lie and tell the truth at the same time.

Can God make a rock that is too heavy for him to lift?

If yes, Can he lift it?

If omnipotence were God's only attribute, the logical limitations might be sufficient; however, other limitations have been found to be necessary because of the many other attributes which people tend to assume that God has. Without these limitations, their definition of God would be logically contradictory and it would be reasonable to conclude that God, as defined, cannot exist.
For example, can God sit down? Although some conceptions of gods in the past allowed for them to be able to sit down, classic philosophical theism has always postulated a non-material, disembodied divinity. Thus, it simply would not be possible for God to sit down - an apparent contradiction to omnipotence, especially since I am capable of sitting down all I want.
To consider another example, is God capable of committing evil? Or, to use a Christian context, can God sin? Once again, some theistic systems have imagined gods capable of all manner of horrible things; philosophical theism, however, has always imagined a perfectly good God. It is inconceivable to believers in such a god that it would ever sin or do evil - even though humans are obviously quite capable of it.
As a consequence, another common limitation to omnipotence which has developed in philosophy and theology is that God can do anything which is compatible with God's nature. Sitting down is not compatible with the nature of a non-material being. Sinning is not compatible with the nature of a perfectly good being. Thus, God may not be able to sit down or sin, but those aren't "really" contradictions with divine omnipotence because this new definition of omnipotence excludes anything contradictory to the nature of the being in question.

The only thing that is omnipotent and omnipresence is Nature and the forces of Nature which is the Supreme God of Gods.

There is no other Gods outside of Nature.

If that isn't bad enough, philosophers and theologians have found themselves devising a number of others limitations upon the definition of omnipotence in order to allow for many more things which God cannot do while retaining the characteristic of omnipotence. A detailed examination of these restrictions is left for another time; what is important to see here is that "omnipotence" has been whittled down bit by bit until there is very little left of the original concept.
Arguably, you and I are "omnipotent" under some of these "refined" conceptions of omnipotence that have become so weak. Any conception of omnipotence which could allow us to argue that we are also omnipotent has become a conception of impotence, especially when combined with the observation that we are capable of many things well outside the ability of this allegedly omnipotent God.
The various conflicts that afflict the concept of omnipotence shouldn't be at all surprising. Philosophers and theologians did not start with an empirical observation that God was omnipotent and then proceed to come to terms with how omnipotence should be understood in relation to God's other attributes. Such a context, if it existed, would perhaps allow us to be more sympathetic to the shifting sand under their feet.
In reality they have began with the religious dogma that God is omnipotent, inferred in part from the religious dogma of God's role as absolute creator, and then proceeded to redefine and redefine and then redefine once more (although these are usually described as "refinements," not redefinitions). This path is followed in an effort to harmonize one religious dogma with a series of other religious dogmas which are also redefined until somewhere, someone hopefully arrives at a set of definitions that don't contradict each other or anything we know about the world, but is still meaningful in a religious context. No one has quite succeeded yet, but that's not for want of trying.
Arguably, the original conception of omnipotence wasn't really coherent anyway - but as more and more of it is removed in order to make it safe for logic and for God's other alleged attributes, we arrive at a position where there seems little point in continuing to use the term at all. Is God all-powerful or just very powerful? The "refined" definition of all-powerful seems to be barely distinguishable from very powerful. The term all-powerful cannot be abandoned for religious reasons, but the concept has been all but abandoned in the details.