The most important things are a decent microphone, some video editing to cut out useless or unentertaining bits (but don't go overboard or it makes things disjointed), and something to add to the experience. If viewers feel like you're wasting their time or they can't stand the audio, they'll leave. Just watching gameplay footage is boring ("I'd rather play the game/watch someone else than watch this guy"). You have to give people a reason to watch, listen, and stay. There are several routes to go about this.
The most typical ways of adding to the experience of watching gameplay footage is to inject your own personality, to be informative, to be really skilled (must know the game in question inside and out), or to do something original. If you choose to just talk and rely on your personality, it's important that your personality be memorable, likable, charismatic, funny, or interesting in some way. If you're trying to be informative, you better be sure your information is correct, delivered as clearly and concisely as possible, and there should at least be some structure to the information (rather than a shotgun approach spewing out tidbits of info at random). If you're relying on your gameplay to stand on its own, you really have to put a lot of effort into being the absolute best you can be so you can amaze people. Simply being 'good' or even 'great' is not enough. You must achieve near-perfection if you want it to be tolerable. Doing something original is the hardest thing to come up with. Good luck with that.
[EDIT: Generating ideas is actually the easiest part of doing something original. The hard part is coming up with something interesting you can do regularly that doesn't require a ton of work. Things that require a ton of work can't be done regularly without huge gaps between releases or a large budget. The former is bad if you want to grow a channel and the latter is infeasible for a new youtuber]
If you plan to talk during gameplay, which is necessary if you're doing something other than the informative or skilled approach, you must make sure your audio quality is decent. If the audio quality is terrible or your voice is grating in some way, people will mute you, close your video, and/or insult you (you'll be insulted a lot regardless of how true their insults are). Maintaining decent audio quality improves the odds that a person won't shut off your video immediately. With a good enough personality and good enough audio, almost anybody is tolerable to listen to.
Ultimately, it's important to hold yourself to certain standards of quality. It may seem like a lot of work, but it can eventually pay off. If you're just making videos for fun and don't have any aspirations for your channel, it can be fine to relax your standards, but don't expect anyone other than die-hard fans (either of you or the game in question) to watch.
