Konchu,
Although the basic needs of trout (water quality/temperature, shelter, and food) don't change much except for the priority of the moment, the way each stream satisfies those needs can be very different. Differences even among streams in the same watershed can be fascinating.
Please forgive my long (probably excessive) response, but I just recommended Tom Rosenbauer's excellent Reading Trout Streams in another thread, and Louis's comments about a stream that we recently shared reminded me of a few of Rosenbauer's interesting observations. I'll paraphrase those comments and suggest the ways in which they match my experience:
Shelter vs. Food--Rosenbauer makes the very reasonable point that the availability or concentration of trout food is often the most important consideration for fly fishers because we generally catch trout when they are feeding. A trout's shelter and its feeding area could be widely separated, though places that offer both in close proximity are usually prime lies defended by somewhat larger trout. The very biggest trout in streams are sometimes exceptions to this. Knowing that a monster trout might be holding on the bottom of a deep dark pool during the day could be of limited value because the fish probably does little feeding there. (A baitfisher's earthworm patiently soaked nearby might prove irresistable, but prospects for fly fishers will be more limited.) That same fish could go on a feeding rampage all over a shallow flat from dusk to dawn, sometimes in water that will barely cover its back.
Overhead Cover--This can be a double-edged sword. Bushes and low overhanging trees, like alders, willows, or live blowdowns can provide protection from diving birds and their shade can aid vision, but higher trees with barer branches can also provide a convenient perch for a kingfisher. (As an aside, I once rounded a bend in a favorite stream only to come up-close-and-personal with an immature eagle as it was taking off from the shallows. We all know that eagles are big birds, but I couldn't have been more surprised had I encountered a pterodactyl.) Although predatory birds are a prime threat, I also fish streams with good populations of mink and even a few that host otters. Sloping rocks or banks near otherwise attractive overhead cover make good approaches for these predators. Little trout in little streams also have to worry about watersnakes and staying out of arm's reach of raccoons. Trout are not the easiest prey for any of these marauders, but those that make dumb choices (including many stocked fish) don't survive long.
Depth--Although any change in depth can be significant on tiny streams, it is often overrated as a place to find trout in larger waters. The greatest depths can provide shelter from the bright sun of middday in summer or a place to rest and avoid the dangers of anchor ice in winter, but fish that take refuge there are often hiding rather than feeding actively. More importantly, fixating on deeper pools can cause us to overlook other, more productive but less obvious places. (Especially ones that are less of a magnet to baitfishers and others.)
Riffles and Flats--I have far too often overlooked rather shallow riffles or flats, only to be surprised by what they held. The very shallow flat (on a smallish freestoner) that Louis mentions above is a perfect example. For years I passed it by, thinking that it only held youngsters. All that changed one day when I half-heartedly cast to a riser in the thin water above the tail. The tiddler that I expected turned into a fat and very strong 16" brown. And this was no hapless displaced stockie, but a savvy streambred fish. (This is a wild trout stream.) On another occasion, I was fishing a broad, nearly featureless riffle on the larger stream into which the above-mentioned stream flows. I had (somewhat grudgingly) surrendered the favorite run below to a longtime fishing partner. My rather random search of the riffle eventually and abruptly turned up a veritable gold mine of large wild browns. Those fish comprised the largest back-to-back string of hook-ups that I have ever had on that stream, and it holds some very nice browns. When I checked, they had all been holding along a long, shallow depression in the streambed that was nearly invisible until one waded into it.
Limestone Spring Creeks vs. Freestone--Rosenbauer says that trout can be "anywhere" in spring creeks. Although that might seem to be a bit of an overstatement, I think it is only overstated in that some of the places that a freestone angler might tend to look first can hold few trout. On classic meadow spring creeks, deep silt-bottomed holes can often be relatively barren, while shallow weedy flats can be tremendously productive. Sections that flow through forested areas often do not hold nearly as many (or as big) fish as the more open meadow stretches. A while back, a very experienced biologist told me that although restoring riparian vegetation is a great idea on most freestone streams, a one-size-fits-all approach to habitat restoration can actually make some spring creeks less productive. He maintained that the open meadow areas of cool, healthy spring creeks almost always hold more and better fish.
My response is already too long, Konchu, so I'll leave it at. I thought your question was interesting.