I have to agree with Frogboy. People's expectations of games have changed drastically since the 80s and 90s. Back in the 80s one of the best games I ever played was Rogue. Today a pure remake of Rogue who be criticized (Read: rip to shreds by almost everyone) as a pathetic Diablo knock off. Or how about Super Pitfall, a remake of that would be ripped apart as a pathetic attempt at a platform game. Even the original Rampage would suffer horribly if someone tried an updated re-release (yes I know there was a new rampage back in the 90s but when you compare the 2 versions you'll see easily it was more than an re-release).
Now 90s games, look at the original Warcraft. That game wouldn't survive the critics with only a 'face-lift'. "Only 2 factions? and they don't even play that differently?" Or maybe MoM as you suggest "Horribly unbalanced." "Basic city building." "Doesn't do anything innovative with the genre." etc... The trouble is game review sites base their reviews heavily on graphics, innovation (that works).
Another problem is a lot of things that worked really well in 2d simply do not work in a 3d game but any game that is not 3d gets destroyed by critics as having 'bad graphics'. Look at the Ultima series for an example. The shift over to 3d was not good for the series. Ultima 8 is none to affectionately called 'Super Avatar Bros' because they changed the formula to integrate some level of 3d-ish appearance. Or Ultima 9 where it went full 3d. Problem is 3d doesn't support parties in rpgs very well.
People also want 'real-time combat' and don't want to 'wait for loading screens'. Look what that did for the Final Fantasy series. The shift to real-time combat without loading screens did not do good things for the series. Although to be fair that series started its downward spiral with the shift to 3d graphics as well. As Frogboy mentioned 3d models are much more expensive and more expensive means less overall content. Take a look at FF6 vs. FF10 for the amount of stuff in the game, size of the world, and places to go. The difference is caused by graphics.
Another factor is probably that in the days of 'poor graphics' there were far more game companies around and so it was a more competitive market. As is always the case competition produces a better product. However today companies like EA buying up all the competition they can reduces competition and as competition decreases so does quality because it becomes "buy our product or don't have anything." The high expectations of the graphics of a game also ensures that Joe Blow can't make a killer game in his garage anymore, at least not one that most people will play.
In a nut-shell in the 80s and 90s most of the development time and money was focused on gameplay because graphics were much more basic. The more time and money spent on pretty pictures the less time and money there is for the actual gameplay part of the game. Ironically many people will refuse to play retail games with bad graphics and yet as Frogboy pointed out they will spend hours on cell-phone games, browser games, and facebook apps which generally have terrible graphics by today's standards because no one expects good graphics from those so as long as the gameplay is good they play it.
Sorry if you disagree but in my opinion the heyday of games was 1990-1998 (give or take a few years) and to a large extent it has been downhill since with a few exceptions.