1. They talk too much.
Of course LPers have to talk; it's kind of how this works. What I'm referring to are the type of LPers who feel the need to fill every single second with non-stop banter, as if they're afraid that if a single moment passes that isn't filled with their "witty" commentary that they'll lose the audience's attention.
This is even worse when the LPer himself has no idea of what to say in the first place, leading to awkward monologues about things having absolutely nothing to do with the game they're supposed to be reacting to. It gets doubly annoying when they are so busy talking over the game that they miss a crucial piece of information, then spend the next 20 minutes stuck on a section that could have been completed easily if they'd just listened to the instructions or clues that they'd talked over.
2. They talk too little.
The reverse is when we have LPers who hardly talk at all. You may get a comment here or there, but most of the video is just the sounds of the game itself. Unless your video is specifically labeled as a walkthrough rather than a Let's Play, I expect you to speak up.
3. They try too hard to be funny.
I don't know what it is about LPers who think that their videos must be part gameplay footage and part standup comedy routine. There are certainly many instances when it's appropriate to crack jokes, but when nearly ever single sentence out of your mouth is a joke, you're simply trying too hard.
The best comedy in LPs, in my experience, flows organically from the action of the game itself. Sometimes things happen that are genuinely funny, or an insightful LPer can make something funny. If it happens it happens, but comedy isn't something that can be forced; and trying to cram humor into your commentary where it isn't needed has the very opposite of your intended effect.
Humor can make an LP enjoyable, but it doesn't have to be your main goal. I've seen several LPers who could have been good if they would just stop trying to be funny all the time and act more naturally. Humor isn't everyone's strong suit. A more serious LPers who occasionally makes a good joke is far more valuable to me than LPers who bend over backwards in their awkward, flailing attempts to make me laugh.
4. They think they can sing.
Don't. Just don't.
5. They ignore the game.
I don't mind if an LPer occasionally goes off topic, but some LPers hardly say a word about the actual game they're playing. This tends to happen more often with LPs involving more than one commentator, at which point it basically turns into those two or more people bantering among themselves with some gameplay footage in the background.
Part of the enjoyment I derive from LPs is observing the reactions of the person who is playing it. When the tragic scene with the death of a main character is playing, I'm interested in the LPer's thoughts on what's on the screen, not his thoughts about his bitch of a girlfriend or where he and his friends hung out last night.
6. The video/audio quality is terrible.
I'm not asking for crystal clear 1080p here. I expect, at the very least, to be able to see what's going on in the game. I also expect the audio to be in sync and of a good enough quality that I can understand what the characters are saying.
I expect the LPer's commentary to NOT sound like it was recorded using a set of tin cans and a tape recorder. I expect the volume levels to be balanced so that the commentary doesn't completely drown out the game audio, and vice versa.
You don't need stunning production values, but producing videos that don't put your audience at risk of migraine headaches may help a little with that whole "getting people to actually watch your content" thing.
7. Too many obscure references.
I don't expect to get every reference a person may make, and it's okay to make jokes that reference some material that not everyone may have seen. However, if the core of your commentary hinges on your audience having an intimate knowledge of some Japanese anime or manga, to the point where much of your video becomes incomprehensible without that understanding, then you need to cool it with the references.
For example, it's okay to say "this guy reminds me of (insert anime character here) from (insert anime series here). While I may not get the reference, I have at least been provided with all the info I need should I someday decide to watch that show for myself. A few jokes related to that character and/or series are also okay. However, if you start using the names of minor characters from that show as nicknames for the game characters, as if everyone is supposed to know what you're talking about, you need to cool it. If you make constant references to some in-show joke that appeared in episode so-and-so of season such-and-such, you need to cool it.
There's giving the occasional wink to fellow fans of your favorite series, and then there's rendering your LPs incomprehensible to all but the most obsessive otakus.
8. They're BORING!
I realize that people have off days, but COME ON! Try to be at least a little enthusiastic about what you're doing. I'm not saying you should be bouncing in your chair and cheering with excitement at every little thing, but at the very least try not to sound like you're half asleep. If you're not enthused about your own videos, then why should I be?
This is not a time for inhibitions. Everyone and their grandmas are doing LPs these days, so that leaves you with one important question to answer: "why should I choose to watch you rather than one of the thousands of other LPers out there?" What do you bring to the table that will hold my attention?
The answer is YOU. Your personality, your commentary, your unique style is what sets you apart and makes your videos worth watching over other LPers who are playing the exact same game. Mumbling into the microphone like you've just downed a heavy dose of Ambien tells me that you don't care and, by extension, neither should I.