EDIT: After reading the original question again, I realized my answer was commenting on the overall thread without answering the original question.
1. I think professional reviewers are affected by advertising and are also trying to make an educated guess as to how well a game will be received based on graphics, sound / voice-over quality, technical issues, and a set of criteria by which they compare games. User reviews are based purely off of how "fun" the game is to the user and how well it meets their expectations of what the game "should be".
2. Users play the games a lot more in depth and either find them to have a magical "fun factor" despite other flaws, or they find the games "just don't work" despite being very high in quality. These factors make them shift their reviews higher or lower than what a professional reviewer might find who may not truly know the genre.
As far as professional reviews go, I only look at them because they often have a comprehensive breakdown of features and gameplay that a user review might not have. They are good for a quick guideline. I don't read as much into the actual review because the bigger a website / company gets, the more they are affected by advertising dollars, especially around the big name titles.
User reviews are usually a better indicator of game quality, but the caveat is you have to actually read through some of the reviews and make sure the people writing it have the same sensibilities as you. There are so many different types of gamers, so while many may not like a particular strategy game, the only reviews that really matter to you are the ones who like the same kind of things as you. Spore is a good example, the game was dumbed down to have broad appeal. A lot of casual gamers really loved the game for what it was, and a lot of hard core strategy people hated it for what it could have been.
Perception and expectations are another factor. Advertising dollars also play a lot into this....they can build excitement in a product that can translate into good satisfied reviews, or negatively with shattered expectations. I think Civ 5 is getting some of the latter right now, though Firaxis will probably improve the product in an expansion to get it back up to the Civ 4 level of quality.
As far as Elemental goes, I was really disappointed, but I have tremendous faith in Stardock to improve the game, not to mention the Stardock community to make some awesome mods for it eventually.