I agree with Steve - the general shape is that of a ships decanter (very much heavier at the bottom for stability). I think this shape is very stylish and the only negative (IMO) being that it's not the easiest to pour from, expecially for the last 25% of the wine. I suppose they also take up a little more space, if that is at a premium.
In general I have simple tastes in decanters and eschew intricate designs and etchings for something clear and practical (albeit a bit of style never goes amiss). A simple glass jug would do the job, but most wine enthusiasts appreciate that a stylish decanter (with Port decanted into it) is nice to be able to plonk on the dinner table at the end of a nice meal.
... and always remember to pass the port to the left, with anyone leaving it sat in front of them without passing it on (when someone else's glass is empty) is clearly related to the Bishop of Norwich
Maybe we should do a 'show us your decanter thread'
regards
Ian