A lot of my favorite games back in the early 90s sucked. I liked them despite them sucking, but they still had huge, massive flaws. I tolerated them then because the market was so small, but I'd be whining my head off about them if they showed up in a modern day, because my own standards have gone UP.
If you want some benchmarks to look at review scores...well, instead of comparing today's awesome RPG against Planescape: Torment (which, be honest, the combat system wasn't that great, D&D doesn't translate THAT well to computer, especially for non-tabletop fans), compare it against the legion of flash games out there, the legion of FREE or donationware little indie games out there, etc.
I posted about
Hero Core last night or so. It's a free indie metroidvania with a retro feel. It's great at doing exactly what it does...but is it objectively as good as Mass Effect, different genre notwithstanding? HELL no. It's short, the graphics are purposefully retro; if I saw it on a review site, I would *expect* it to get a score in the mid-high 60s, with a summary like "A decent way to spend a couple hours if you're a fan of the genre". It has VERY polished gameplay, but that's the score it deserves in my mind.
What deserves lower than a high 60? Well, I still think that modern hits shouldn't be compared scorewise with other modern hits, but instead, with all games regardless of length. I thought that
Today I Die was a neat little artsy flash game. Great presentation, but hey, short, confusing, and at the end, you shed a single tear and then forget about it entirely. It's not a bad game, it just has zero lasting impact, it's confusing...I still liked it, but it does not deserve a high score. What does it deserve? Low 40s for a game you can play in five minutes (or 30 seconds if you know what to do).
There was some overhyped game called Time Fcuk on Newgrounds a while back. You could spend MANY hours on it, if you found it challenging, and if you liked playing with the level editor and sharing levels with others. But I really didn't like the execution at all, I didn't enjoy the game. Played it anyway because I was bored, but regardless of length, it gets a low 40s from me as well.
There's a ton of shareware-type games out there where you click to find objects on the screen, or to find differences, or whatever the hell. Some people like them, like my ex, but the graphics aren't great and the gameplay is quite middling. But they still do a "play for an hour free, then buy it"--so it's not just free games that I think are bad. Low 60s from me.
Today's tower defense faffle? Probably in the 50s. I could blow some hours on it, but I'd feel dirty afterwards.
What ranks in the 20s-40s range? One of those weird semi-Rube Goldberg games on Newgrounds, where you click the lever and the windmill pops up, then you click the windmill and it blows the seeds over to wherever and they start growing and open a door and whatever. There's some really bad ones out there.
Below 20s and it's not really playable but YOU CAN STILL FIND IT ON NEWGROUNDS, and the players still rate it 10/10, because Newgrounds users have terrible taste. I saw several people give a rating of 10/10 to a giant mecha dressup "game" (click a part of the mecha, and it cycles through a library of stuff). The sounds were way oversampled, the graphics weren't that great, and there was no "game" at all. It didn't even deserve a 10/100.
While I'm at it. The indie games that are a lot longer, a lot more worth playing, that I think EVERYONE on this board should play? Iji, which can easily eat up a week or three... High 70s, not quite low 80s. Imperfect controls, graphical style got a B for effort but a C for execution, the story really reached and did a great job--but it didn't follow through with all the promises of storyline branching that it sold to the player. It was a wonderful game. But there's no way it deserves a score in the 90s. Spelunky, which many players have an addiction to? I'd put that at low 70s. An awful lot of big-team games are better.
In all honesty, I probably overrated many (but not all) of these indie games. Ask yourself, "What would this indie game have to do to get a score in the 80s?" If the answer is "a hell of a lot, like better graphics, bugfixes, better controls, more game modes, a longer story, a broader story, and hell, why not multiplayer", here's a hint, it doesn't deserve a score in the 70s. That puts a lot of fun, quite-replayable, small-budget games in the 60s. (But not Iji, which is great.)
So, in summary.
NO. Standards are NOT slipping. Modern big-budget games, when compared against all the other games out there, are pretty darn good.