I watch LPs pretty much all day, so I think I have a pretty good grasp on the medium. And really, it depends on what you're going for.
Are you the role-player, like Rythian of the Yogscast? If so, you have to be very invested in the games you play - immerse yourself completely, and make sure you play games that focus more on story than anything else. You have to pretend you ARE the character.
Are you the joke-maker, like Jesse Cox of OMFGCata? If so, you must always be ready with witty things to say, and if you can't pull that off, you may as well flush your series down the toilet.
Are you the multi-player, like Duncan, Simon, and Lewis of the Yogscast (they tend to play together a lot, at any rate)? If so, make sure your friends are somewhat varied in their personae. For instance, if everyone you're playing with is a massive troll/noob/stoner, your series will suffer. Have a friend for each (or whatever other personalities you want, not necessarily those negative ones), and take the chemistry into account. Just because YOU laugh at everything your one friend says, doesn't mean the audience or other members of your party will respond in the same way. The coop experience in LPs should make the audience feel as "in" with your group as you are.
Of course there are other ways you could go, like the walk-througher, who shows you how to complete absolutely EVERYTHING in a game and will show you cut-content and hidden lore and all that. And you can cross into multiple types of any of these fairly easily, so long as you know what you're doing.
Another tip: don't try to BE somebody else (i.e., acting ridiculously annoying like PewDiePie or some other famous YouTuber). I only listed the people above as examples; a template through which you may critique and tweak your OWN work, not mirror theirs. There are millions of LPers out there who are trying to be somebody else and they ALL suck.
And remember: if you're not having fun, the audience isn't either. If a game is too frustrating, or you simply can't say enough about it to keep the commentary interesting, just move on to another game.
There's also the general stuff that's true of any uploader: make sure your mic is HIGH QUALITY. NO EXCEPTIONS. Always upload in at least 720p, and make sure your video fits the screen properly and the like. You want your stuff to look as professional as possible.
I always kind of wanted to dabble into LPs and game reviews myself, but I just haven't the time or proper working environments right now. After college, it may be a different story, but then again I may be too busy MAKING games to review them. :/
If I was to do an LP, it would be a cross between the walk-througher and the joke-maker, for sure.