For me, it has to be something of a combination.
While I wouldn't say comedy in and of itself is important, the LPer has to have an engaging personality.
The game, of course, needs to be something I actually am interested in.
The LPer should have some knowledge of the game going in.. they don't have to be perfect by any means, but if I'm watching a LP I don't care to see someone dying at the same part 63 times in a row. I just don't find that entertaining. If you end up having trouble with a part, don't be afraid to edit the video to make it more watchable. For example, show the first couple of deaths, maybe a really close attempt or two, and then the successful run.
If I'm watching a full LP of a game, I don't really need it to be from a review style perspective, but actual commentary about what is working in the game and what isn't is a plus. Also, try to bring something extra to the experience like some personal knowledge, personal rumination on characters/plotlines, etc.
I'd say everyone has their own tastes in an LP, but it's most imporatnt to bear in mind that there are literally dozens, if not hundreds, of LPs out there for bigger titles. In some ways it's less important to worry about how exactly to make your LP stabd out than it is to just make sure that it does stand out.