It's important to keep in mind the full quote (see below):
We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.
Yet we should not pass up our opportunities in that critical 3%.
What this means is that, in the absence of measured performance issues you shouldn't optimize because you think you will get a performance gain. There are obvious optimizations (like not doing string concatenation inside a tight loop) but anything that isn't a trivially clear optimization should be avoided until it can be measured.
The biggest problems with "premature optimization" are that it can introduce unexpected bugs and can be a huge time waster.
There is no doubt that the grail of efficiency leads to abuse. Programmers waste enormous amounts of time thinking about, or worrying about, the speed of noncritical parts of their programs, and these attempts at efficiency actually have a strong negative impact when debugging and maintenance are considered. We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.
Yet we should not pass up our opportunities in that critical 3%. A good programmer will not be lulled into complacency by such reasoning, he will be wise to look carefully at the critical code; but only after that code has been identified. It is often a mistake to make a priori judgements about what parts of a program are really critical, since the universal experience of programmers who have been using measurement tools has benbeen that their intuitive guesses fail. After working with such tools for seven years, I've become convinced that all compilers written from now on should be designed to provide all programmers with feedback indicating what parts of their programs are costing the most; indeed, this feedback should be supplied automatically unless it has been specifically turned off.
