Four things that affect the flavour of Whisky

Bill Hutcheson gives a great primer on what affects the flavour of whisky here:

This is the main extract:

Three things affect the flavour of malt whisky, four if you include peat.

The first major impact is the still. What shape it is, how tall it is and what angle the lyne arm is set at and so on. All these 'still' related factors directly impact the quality and nature of the 'new spirit'. If the still is short then we get a robust oily spirit, if the still is long and thin we get a fine lighter spirit. The lyne arm is the bit that takes the spirit from the top of the still to the condensers where is becomes a liquid again. The angle of this device affects what its called 'reflux'. Also affecting reflux action are the bulges in the still, dents and any other shape change in the still.

Reflux action is when the vapours get so far up the neck of the still but then fall back into the still to be re-distilled again. The more reflux action the more refined the spirit.

The second and biggest impact on the flavour of malt whisky is wood. Malt whisky is put into white oak casks for maturation. These cask are generally ex-bourbon casks, but can be sherry or wine. The characteristics of American white oak are completely different to European oak and this you can taste!.

American oak offers vanillas from the wood sugars, can also get toffee. The effect of this wood diminishes each time the cask is used and they usually only use a cask 3 times.

European oak is completely different, offering fruitier flavours, and if a sherry butt you can get raisins and fruit cake.

'Wood finishes' are when a whisky has been matured in an ex-bourbon cask but is then 'finished' of a few months in another type of cask, and this is what Glenmorangie do. The spirit is fine from their long stills and is smooth. However it's such a fine spirit that finishing it in another wood gives it extra character, some might say making it less bland - I wouldn't of course. Other finishes can include, Marsala, Port, Rum and many wine types.
Be careful though it doesn't always work and the wood can sometimes be too much for the whisky.

The final impact on whisky is 'Time'. If there is one thing a whisky maker must have its plenty of time!. Whiskies have an optimum time, as they can be too young or too old. The skill of the master distiller or blender is to select the casks at the right time. This is different of each individual cask and a difficult job, expensive if he gets it wrong.

Never assume that the older the whisky the better the whisky - I've tasted some pretty mediocre £300 a bottle 30 year old whiskies and some fantastic 7 year old whiskies. Whisky can be over-oaked just like wine - especially Australian wine!