I'd say mostly they're a bit too irregular. Plate boundaries are a model, and in reality the boundary has finite width.
There are three types of plate boundary, and they all look different on a map.
Ridges have the most distinctive shape. They comprise a series of ridge segment aligned in one direction, offset by transforms at right-angles. Most of the offsets are small, but you get some really big ones. You can see that clearly in this map of the Mid-Atlantic Ridge: http://cimss.ssec.wisc.edu/sage/geol...epts_fig15.gif
Subduction zones where one ocean subducts under another often have a curved shape, with the subducting plate on the outside of the curve. The Aleutian subduction zone is a good example. Where an oceanic plate subducts under a continent the subduction zone will follow the general shape of the continental margin. Not all subduction zones have ocean trenches - if the ocean floor is young then it's less dense and doesn't sink to form a trench, and if there's lots of sediment coming off a continent that will fill in the trench.
Strike-slip faults are usually pretty straight, though they can have some elongated S bends. On Earth, excluding the ocean ridge transforms they're relatively rare. The San Andreas is the well-known example, there's also the North Anatolian Fault in Turkey.
All this applies mostly to oceans. Within continents there can be deformation over large areas and plate models may not apply.
The problem for creative mapping then is that you can't determine the shape of plate boundaries properly without also considering their motion. Then there are constraints on that motion. In particular, there are some stringent constraints on the orientation of the boundaries and the motion of the plates at triple junctions (where three plates meet).
I'm working on a tectonic world myself, and it is rather difficult. Simply picking random motions has left me with a large number of transform faults - while I can handwave that issue away since it's not Earth, it's still a bit annoying. The bigger problem is I have loads of impossible triple junctions. (But I'm going with it anyway, rather than start over.)
(A sidenote about your plate centroids - if you create latitudes and longitudes at random, you don't get a uniform distribution over the planet, since 1°x1° is a much smaller area near the poles that at the equator. This probably doesn't really matter, but bear it in mind.)
EDIT: Ah, I see you do have directions.
You can get small plates surrounded by multiple others, but not a plate as an enclave of just one other.
Iceland is a thickened part of the Mid-Atlantic Ridge. The plate boundary runs through it.
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