Some quick background is in order. Until the late 1970's, when Robert Parker decided to drop his law practice to start Wine Advocate and provide independent, objective wine criticism, there wasn't much if any independent wine criticism. It was a common thing for wine critics to be financially involved with wineries and other parts of the industry; not only that, but wine reviews from that era rarely aimed for any kind of objectivity and often provided little to no information about the wine's quality.
Whatever else you can say about the subject, the introduction of an independent wine critic was a fundamentally revolutionary force for the industry. But it didn't happen overnight. The 1982 Bordeaux vintage was panned by most if not all major wine critics; Parker alone went out on a limb, identifying 1982 as a banner year for Bordeaux. When the vintage was released, Parker's status sky-rocketed; since then, Parker has become the biggest single-person brand name in history. His rating systems are criticized precisely because they are so influential; a rating of 90 vs. a 89 can be responsible for millions of dollars in sales and ratings of 95 points and up can create such high demands that some cult wines auction for thousands of dollars a bottle (or at least they did prior to the global financial meltdown).
Dave Nelson isn't the only person criticizing Parker's stylistic preferences; the criticism is often made that Parker's preference for big, massive wines loaded with fruit have driven winemakers all over the world to 'Parkerize' their wines, ie, make their wines in the style that Parker likes as it's easier to get a good rating that way. But the picture isn't so simple; while it's true that Parker does influence the stylistic decisions of many winemakers, the net effect is that wine around the planet has gotten better. The wines of Bordeaux and the Rhone particularly are good examples of this: Parker's reviews and ideas have pushed winemakers to understand the science of winemaking, to use clean equipment and better their vineyard management processes and to find the true potential of the grapes that they grow.
As a consumer, I'm glad of that. Here is Parker's rubric for scoring wines:
In terms of awarding points, my scoring system gives every wine a base of 50 points. The wine's general color and appearance merit up to 5 points. Since most wines today are well made, thanks to modern technology and the increased use of professional oenologists, they tend to receive at least 4, often 5 points. The aroma and bouquet merit up to 15 points, depending on the intensity level and dimension of the aroma and bouquet as well as the cleanliness of the wine. The flavor and finish merit up to 20 points, and again, intensity of flavor, balance, cleanliness, and depth and length on the palate are all important considerations when giving out points. Finally, the overall quality level or potential for further evolution and improvement—aging—merits up to 10 points.
Scores are important for the reader to gauge a professional critic's overall qualitative placement of a wine vis-à-vis its peer group. However, it is also vital to consider the description of the wine's style, personality, and potential. No scoring system is perfect, but a system that provides for flexibility in scores, if applied by the same taster without prejudice, can quantify different levels of wine quality and provide the reader with one professional's judgment. However, there can never be any substitute for your own palate nor any better education than tasting the wine yourself.
That's not so bad as an effort at an objective evaluation of a wine. But as far as a consumer cares about what they're drinking, context is extremely important and point ratings alone don't tell the whole story.