I'm going to attempt to clarify something that appears to be confusing about the essence/energies distinction, particularly for people who aren't familar with St. Maximus Confessor's discussions on logoi, tropos, and the gnomic will. As simply as I can state it, St. Maximus distinguishes between a faculty itself and the use of a faculty. The faculty itself is a property of nature, while the use of the faculty is personal. The most obvious example, and the one St. Maximus uses most frequently, is the will. Having a will capable of choice is a property of nature, but the actual choices made by the will are personal. Similarly, God has "properties" (a term which should be understood as applying only analogously to God) that are intrinsic to nature, such as the Father begetting the Son and generating the Holy Spirit, and other voluntary decisions that are Personal (although owing to the mutual love of the Persons Trinity, such decisions are always harmoniously expressed by all three Persons). Thus arises the difference between the necessary (Persons of the Trinity) and the voluntary (creation) (which incidentally traces all the way back to St. Athanasius's defense of the Trinity). The voluntary creation is a choice of God to express Himself in some of the infinite number of possible ways in which God can express Himself, which are the divine logoi, quite similar to St. Thomas's "divine ideas." Note that in analogy to humans, the sphere of choices is defined by nature (although in God's case, the logoi are infinite), but the actual choice is voluntary.
There are several notable consequences of St. Maximus's theology. One rather obvious one is that Calvinism or any similar philosophy that attributes necessity to all choices can be rejected out of hand. Free will is essential for a coherent theology; if you attribute necessity to all God's actions, then you become a pantheist (viz., you are maintaining that we are all necessary to God's existence). Moreover, on the soteriological side, the presence of the logos, the imago dei, within each human being, is the very basis of recapitulation in Christ, the essence of human salvation. A somewhat less obvious corollary is that the notion of actual guilt for original sin is untenable, because guilt cannot possibly accrue to nature (which is an expression of the divine logoi and therefore good by definition). That's an issue that tends to get glossed over in the West when discussing the Immaculate Conception, because we don't tend to see the "big deal" in attributing guilt of original sin to humans, not perceiving the danger in inadvertently attributing evil to God Himself (or conversely, denying the basis of salvation).
Thus, one can see the difficulty in the question posed by Elliot:
If God necessarily, and by essence, exists in his energies, then what determines *which* energies, and thus which actions, God manifests?
This is tantamount to a denial of God's free will, since it says that for God has no choice in expression of Himself. The entire point is that energies are a Personal decision (and really, Tri-Personal, although particular expressions may be identified with one Person in terms of "mission") in how God expresses Himself from among the infinite possibilities. The essential part of God is not that He has energies, but that He can have energies, much like the essential part of being a free being is not choices, but the ability to make choices. You've essentially collapsed the metaphysical distinction between person and nature in your question. If you consider Person a metaphysically real distinction, then this isn't a problem. Unfortunately, the most common interpretation of St. Thomas (although by no means necessarily the correct one) argues that the personal distinctions are not metaphysically real, which leads into a whole host of difficulties, such as the ones that Hughes outlines in On A Complex Theory of a Simple God.
Alas, even that explanation doesn't even get into the real heart of the essence/energies problem, which is the knowability of God. The real reason for distinguishing between the essence and energies is the Eunomian problem: if essence is absolutely simple, and one knows the essence, then one becomes the essence (one is God). Therefore, you have to distinguish between knowing the essence and knowing the energies (with "knowing" being a more comprehensive term than simple knowledge) to avoid any union with God involving absorption into the divine essence (the classic problem of the Neoplatonic One).
The reason the West got crossed up with the East on this point was that they encountered the problem of knowability in different contexts. The milieu of the Eastern conflict was the Neoplatonic problem and the struggle with the Eunomians, but in the West, it was the conflict with the Arians. The Western Arians raised John 6:46 ("Not that any one has seen the Father except him who is from God; he has seen the Father.") to argue that the visibility of the Son distinguished the Son from the invisible Father (viz., the Son did not share a property of the divine nature). St. Augustine, based at least partially on his understanding that Matt. 5:8 promised an actual vision of God, argued that the invisibility here resulted from the veil of creation that would be lifted for the "pure of heart" who would "see Him face to face." Consequently, the fact that humans saw God in Christ did not violate the invisibility of God any more than did the theophanies of the Old Testament (which St. Augustine construed as created effects from which the presence of God could be deduced). "Seeing God" in the sense of rationally deducing God from the effects of creation could thus be analytically separated from the physical perception of God, which was associated with glorification.
The problem then becomes apparent: the Eastern view says that God is revealed in uncreated energies, which *are* God, while Augustine says that He is only revealed mediately until the eschaton. My hunch (although I haven't seen anyone argue it explicitly) is that the real problem is equivocation in the term "to know." St. Augustine seemed, at least implicitly, to include the limitations of analogical thinking in his account of reason and knowledge. Particularly, I am thinking of his hierarchy of how God is reflected in creation, from vestigium to image (reason) to likeness (grace, regeneration). ISTM that his solution to the Eunomian problem would simply be that we never "know" God in the sense that the Eunomians mean it, but only according to some mode of revelation, which progressively increases in intimacy while never reaching actually being God. One might analogize the Eastern view to a path that brings one infinitely close to the "black box" of the divine essence, while St. Augustine's view is more like the divine essence expanding outward until it is infinitely close to the people at the periphery. The problem I see with the latter formulation (although it is not insurmountable) is that both Christology and soteriology are less intuitive. The essence/energies distinction almost works too well in that regard; the tendency is to say "if it ain't broke, don't fix it," leading to parochialism. The Western view, on the other hand, is extraordinarily difficult to explain systematically. You have to manage essentially to tie the entire metaphysics back to the human perception of reality, and Xavier Zubiri's formulation of reality as structure is the only one I've seen that does that (and to express my parochialism, I think it repairs some problems in the Greek metaphysics as well).
I'm not sure that I actually answered any questions, but when somebody asks me for my thoughts, they've got to take the chaff with the miniscule amounts of wheat. :-) To put a brief conclusion on it, my position is that the essence/energies distinction is a viable philosophical position for Christian orthodoxy, but I don't think it's the only viable position (contra some polemical Orthodox Christians), and when push comes to shove, I prefer the Zubirian account.
