Shiraz and Syrah are wines of power and intensity, both have blackberry-like black fruit characteristics, medium to strong tannin structure and a touch of black pepper. However, the climate in which the grapes are grown will make a significant difference to the flavour of the wine. In a cooler climate Shiraz ripens slower and produces wine that are more medium bodied and with less tannins.
In warmer climate such as the Barossa Valley in Australia, the grapes ripen fully and the resulting wine will be bold, full-bodied, spicy with dark blue and black fruits, with powerful flavours and often a chocolate note. This type of style is considered the “classic” Shiraz. With riper fruit often comes a higher alcohol level.
Structurally the wines generally have moderate acidity and medium to strong tannins. Which climate it comes from does not alter this and the winemaker has a great deal of control over it, so you can find each wine will have subtle differences under the winemaker’s influence.
People often group wines into either “Old World” or “New World”, the definition can be simplified to anything produced in Europe is Old World wine and anything else is New World wine, although it is a little more complex than that.
Old World Shiraz typically means wine hailing from cooler climates so they will be leaner and more reserved than New World Shiraz with a lighter body and less fruity flavours. It’s still possible to get Old World Shiraz from some parts of typically warmer climates like Australia. Cooler climate Australian Shiraz is more similar to Syrah from the Saint Joseph region in France and will have more earthy notes.
The reason people have associated Shiraz as bold and powerful is worldwide Australian Shiraz has generally received most of the acclaim and high point scores. Since these were from warmer climates the powerful flavours shone through and that style became well known.