Well I was generalizing, because that's what a Let's Play is. Sure, there are different types, but for a Let's Play to be a Let's Play, it just has to be someone playing a game and then posting it on the internet. It doesn't have to be good to be a Let's Play, it just can't be...say, a clip or highlights. It has to follow the player.
For instance, someone can start a game and play it to the finish with absolutely nothing to say, it'll still be an LP. But if...say, Game Grumps do a one off, it's still an LP. Similarly, if someone does a challenge or has some type of quirk to the LP, it's still an LP. But it's because of the broad nature of the genre that it really doesn't interest me as a whole. And saying that there are certain prerequisites for what constitutes as a "proper Let's Play" is more a matter of personal preference/qualifications than an objective identification of what counts and what doesn't.
The whole reason that I got into Game Grumps wasn't because they were LP, I rarely even have a chance to refer to them as that. In fact, the appeal there is mostly the commentary and not the gameplay (though they do go hand in hand a good number of times). I got into it because I saw that an animator I knew of growing up (or...in my early teen years forward, I guess) made a channel and it ended up being pretty funny. That was assured quality, basically. At the same time, and I'll 180 this back to my original point, they're not the majority. They can't be. LPs are too broad to be and, as a concept, too simple for them not too be done by the masses. It's basically the same reason why I don't watch something on youtube made by some random Joe and expect it to be good, especially when taking views into consideration.
And in the case of games I won't ever play...I probably won't watch at all. With GG, it works because I watch for the commentary first (but of course, gameplay and other facets aren't counted out of the equation completely, especially since I watch the show and don't treat it as if it were a podcast). There are other reasons as well, but that's the sort of idea to the show, for the most part. That's why the appeal is so varied, especially because they never ignore the game altogether and don't really bind themselves to specific genres. And I know not all LP should be about the commentary first and foremost, but it works to view GG that way because that's their hook. It's in their name. But if you're watching an LP that puts the gameplay first, unless the LPer is doing something explicitly entertaining (scare-cams for horror games, commentary/challenges/etc for narration non-heavy games, etc.) then I can't see any reason to watch. Because if I don't want to play the game and the LPer doesn't really provide to make up for the "why", then I'm just watching the game. The game that I don't want to play. And in the case that it's a game that I can't play for whatever reason, I still wouldn't, because it would be riddled with spoilers for when the chance comes for me to actually play it. That's just me, though.
But I think the biggest difference is you go into them expecting them to be good, but I can't do that for anything that's so easy to produce. Anything at all; because when that's the case there's such a large margin for quality.