I can knit any sweater from the top-down by using one of three basic garment plans: raglan, circular yoke, or shoulder-down.
The Peach Vine Pullover, a shoulder-down design
With a raglan or round-yoke sweater, you cast on the neck and knit the body and sleeves in a continuous circle (or back and forth if you're knitting a cardigan), increasing as you go. In a raglan, the increases are placed at the points where the body divides from the sleeves. In a round-yoked sweater, the increases are evenly spaced around the yoke.
For a shoulder-down sweater, you cast on the back, knit a couple of inches, pick up the front shoulders, knit the front down a couple of inches, pick up along the edges for the sleeves, and then knit the body and sleeves in much the same way that you would for a raglan.
Dragon Princess Shell, a shoulder-down design
In general, a raglan or round-yoked sweater will have a less fitted, more casual appearance than a shoulder-down sweater. A shoulder-down sweater hangs from the shoulders in a straight vertical line while a raglan or round-yoked sweater radiates out from the neck.
Hunter Rib Sweatshirt, a raglan
Raglan and round-necked sweaters are easier to design and knit than shoulder-down sweaters. A raglan has strong diagonal increase lines from shoulder to underarm. These lines can be a design point in a stitch pattern with a strong vertical element. You can put cables inside the raglan lines to emphasize this diagonal line. In other stitch patterns, the raglan lines create an unwelcome discontinuity.
Spanish Tile Cable Sweater, a raglan
Raglans are an especially good choice with ribbing and other stitch patterns that tend to stretch horizontally more than vertically. With stitch patterns that tend to stretch vertically, the shoulders and neckline will tend to stretch out of shape with a raglan.
Raglans are a good choice with wool and firmly knit cotton yarns, but might not be such a good choice with linen or silk blends or other yarns that tend to stretch out of shape.
The pony sweater, knit as a raglan
And with a round-yoke
Round yokes are used mostly when the pattern has a strong horizontal element that you want to continue uniformly around the yoke and you have the occasional plain round where you can stick a bunch of increases. Between the 3-5 increase rounds, you work the pattern uninterrupted. Round yokes are often used with color work, but can also be used to good effect in brocade, lace, or other pattern stitches.
The round yoke of Malcolm's Cat Sweater
A shoulder-down design works well with vertical patterns that you want to continue straight up to the shoulder and with designs where the sleeves have a different pattern than the body. If you want to knit a sleeveless garment such as a vest, shell, or tank top, you'll need to use the shoulder-down approach.
A shoulder-down tank top
A shoulder-down design has better stability in the shoulders, neckline, and sleeve caps. This makes it a good choice for nonresilient yarns, particularly those knit at a loose gauge. Shoulder-down designs are by far the trickiest top-down design to plan and knit, but they also yield the best fit.
When I first started knitting top-down, I knit everything as a raglan. Most sweaters work fine as raglans, and I knit them exclusively for almost 20 years. The first shoulder-down sweater I knit was a tank top, followed by a few saddle-shouldered sweaters with shoulder straps. Over time, I came to prefer the fit of shoulder-down sweaters and now knit more of them than anything else.
A plaid pattern, knit shoulder-down
I didn't learn about round-yoked sweaters until fairly late in the game. Traditionally, they're used in Icelandic sweaters, and I didn't do much color work. Now I use the round-yoked design any time I have a strong horizontal element in the yoke that I want to work continuously. They're somewhat more work to plan than a raglan, great fun to knit, and the resulting color work is usually quite popular.
The plaid design used in the previous sweater, modified to work with a round yoke