'Images and Words' is often hailed as a milestone of progressive metal. In reality, though, not only is it hardly progressive, it is less than decent in quality. Okay, it is "progressive" - in the sense it sounds like what is called progressive these days. Strictly, though, it is little more than an incohesive combination of power metal, Rush, and pop that allots more room to pop than power metal.
The pop allegation is the one that would raise the most eyebrows, but poppy sections are certainly more abundant than power metal sections. While he sounds a bit hysterical, LaBrie is a decent singer. However, the vocal lines he sings are not decent. Sometimes they are - in Pull Me Under, for example, he has some power metal lines, though the "power metal" here is closer to Helloween than Fates Warning. But most of the time his lines are poppy whether he sings quietly or loudly. The quieter songs here hardly sound like metal for the majority of their durations, and the decent power metal moments are sparse.
Song structures are not as simplistic as pop, but they are not the advanced progressive grandeurs that one might expect from the hype (look at 'My Journey to the Stars,' for a 1992 example of real structural brilliance). Transitions are awkward, structures amorphous, and development barely present. The Rush-inspired, sometimes not only in principle but also in the specific form of melodic execution, instrumental sections have little if any relationship with the song at hand, and are seldom consistent within themselves, feeling more like a rock band jamming on stage than a thought-out composition.
As it would be expected on this type of wandering but essentially easy-listening album, there is no emphasis on the riffs. The riffs are mostly some chugging and chords, when they are present at all. There are some little technical instrumental interjections, but these, rather than complementing the main themes, work to interrupt them, as in Under A Glass Moon, serving to prove at once both the band members' high instrumental proficiency and their poor songwriting ability. Even during the long instrumental sections the keyboards generally take the lead role, leaving little room for riffing. Overall, the instruments and the vocals are not integrated well. There are vocal sections and then instrumental sections, working separately (i.e. the instruments seldom do anything meaningful while the vocals are on display, and vice versa) and not even continuing on with what the other has been doing but starting something irrelevant.
Another attempt at progressiveness resorts to the old, useless trick: diversity. From the jazzy saxophone usage of Another Day to the almost rapping vocals of Take the Time, none of this trick adds anything of substance to the songs. Unfortunately, this is an aspect that many bands took influence from, perhaps because it is one of the easiest ways of sounding prog without real effort (or being actually progressive).
'Images and Words' indeed was a highly influential album, setting many connotations of the term "progressive" today as it is used in metal music - lack of integration, flashy instrumental techniques, and variety of transparent influences irrelevant to the rest of the music (note that "innovative artistic achievement" has not been mentioned). These are characteristics exhibited not only by the style of music directly under the influence of Dream Theater, but also by extreme metal bands, especially former black metal bands that claim to have evolved. While I do not hold Dream Theater solely responsible for this, I curse it nonetheless.