Now, Craig - a few points before I do one for spinners.
1, it's not just Test-cricket, it's all First-Class-cricket. Indeed it's also any cricket of limitless overs - Grade Cricket included.
2, as has recently been pointed-out, you can only swing a ball when it's in the right condition (hoop it conventionally when it's shiny enough, reverse it when it's reversing) but a good bowler simply has to be able to do both. Off-cutters and leg-cutters are effective on any surface but there's no need for them if the wicket is seaming, as they're harder to bowl than simply getting the seam straight and it's easier to misdirect.
3, without a good seam-position you're not going to swing a Kookaburra ball, and you're not going to seam any ball as often as you would. The reason a Kookaburra ball encourages better bowling habits is that you've got to get the seam absolutely straight. A Duke ball will swing if the seam's just roughly perpendicular.
4, finally, no-one will recognise this. If you discuss bowling, especially these days, the first word on any supposed sage will be 90 mph pace. If you can bowl at 90 mph as supposed to 80 (or 145 ks as supposed to 130), you will automatically be classed above the rest.
Sideways-movement is almost always the last thing to come to the mind of almost any credit for "good" bowling. There really is a fixation amongst cricket sages about pace ATM. Especially, it seems, in England.
Now, what makes a good spinner in limitless-over cricket?
IMO, it is this.
First of all, what is a "spinner"? In all honesty, it is simply someone who generally bowls at about 50 mph.
The first and foremost neccesity for a spinner is the ability to turn the ball. If you can't turn the ball, you're highly unlikely to be effective. The simple physics of the human body dictates that to spin the ball with your fingers doesn't impart enough spin to turn the ball except on what I regard as an extravagent pitch. Hence, a fingerspinner, even if he bowls a "doosra" as Saqlain and Harbhajan do, simply cannot be effective (given competant batting) except on pitches typical to most subcontinent and West Indian grounds, Wantage Road, Sophia Gardens, The SCG and Basin Reserve.
So, to turn the ball sufficiently on any surface, you need to spin the ball with your wrists. Most bowlers who have done this down the years have had problems with dragging the ball down about every other over. The exceptions have been Sydney Barnes, Clarrie Grimmett, William J. O'Reilly, Richie Benaud and Abdul Qadir.
Recently, we have had 4 wristspinners who haven't had this problem. Their names have been Kumble, Warne, Mushtaq Ahmed and Muralitharan. Murali is not by any standards an orthodox wristspinner, neither is Kumble. Kumble isn't really that good a Test bowler except on extravagant surfaces, but he is much quicker than most spinners.
However, there are other attributes that contribute to the skill of a slow-bowler. None of them make-up for the lack of ability to turn the ball, but they do all conpliment turn very well if you can do them.
The first is what is known as "drift". Drift for a spinner is similar to swing for a seamer, though not exactly the same as swing can be effective without seam. It is simply the movement of the ball through the air. Like swing, the later it moves, the better. Whereas swing is achieved by a good seam-position and well-placed shine on the ball, a ball will drift if you flight it in a certain way and is certainly exaggerated by wind speed and direction. The arm must be slightly angled at delivery for a ball to drift and the best spinners vary slightly their arm angle to vary drift.
Loop is another technique used by spinners. There is no equivalent for seamers. It is simply the ball dipping just before pitching, so ending-up a shorter delivery than the batsmen has first thought. It is especially useful if you can draw the batsman into a drive to what he thinks is a Half-Volley but ends-up just short enough to catch the edge with turn. Loop is achieved simply by lobbing the ball up into the air with less pace than normal. It's not the most complex of techniques, but it still takes nous to know when to do it.
Finally, a point on all bowling:
It is all very well to be able to move the ball in any way you like, and to be able to hit a spot you're aiming at 19 times out of 20. But you still need to know when to bowl which length, and when to move the ball and when to keep it straight. Often, people make the mistake of saying "that's inexperienced bowling" when someone bowls a certain delivery. Often, it is simply that they have been trying to bowl something and it's come out wrong. It is an insult to the nous of a bowler to suggest he doesn't know what to bowl.