No one asked me, but I have an over-inflated opinion of myself* so I'm going to offer some of my own tips too:

In the Transitions sections Rockslider mentions using a cross-dissolve (otherwise known as a cross-fade). Personally I don't really like these as it can look pretty bad with game footage (high contrast and busy scenes). I find it more elegant to fade through black if you don't want to do a hard cut. I often use fades through black with a total time of about 0.6-0.8s.

With regards music fades on cuts, you don't have to fade the music at the exact same time and speed as the footage. I often have the music begin to fade out a second or more in advance of the footage beginning to change.

Speed changes are best when the footage is fairly steady moving-through-space. i.e. a Warthog ride or a jog somewhere. You speed it up to preserve the continuous flow of the footage, to show 'every step of the way' and ensure that people know where you've gone in the game-world without having to show it in real time. It does get harder to watch during fighting sections where the POV is snapping around to different targets so as Rockslider says make sure you know what you're doing. You can also fairly safely speed up footage where you're just waiting in place and your view doesn't change much.

Alternatively, you can use that downtime to explain earlier tricks in more detail, tell people about alternatives or outline what people are about to see next so they're ready to have their faces melted. Or you can just make idle chat - which is especially effective in two-person commentaries - but don't go too off-topic.

Captions wise, people read descriptions at a shockingly low rate. The video needs to speak for itself. Either with on-screen text or literally. Sometimes people disable features like captions wholesale due to others misusing them, so only make such content additive and not essential.

Occasionally, you can use very quick on-screen text to inject humourous comments. That is the only acceptable use of too-fast-read text in game guide videos. But again, additive, not essential.

Voice-overs are also an easy way to add a signature/watermark to your work. Opening/closing logos can be edited out, corner logos can be blurred, cropped or edited over. But your voice embedded in the audio stream makes it pretty obvious who made it. That can't be removed easily without removing the audio entirely - which can ruin the video.

Action games can be really loud. You'll have to play with the leveling for each individual game, but Halo is mastered REALLY loud. I generally reduce the volume by -16 to -23dB in my editing software to give my voice good breathing space. If the game has the feature, and to save yourself a little editing time you could reduce the volume in-game (and then turn up your TV volume as you play) - but you'll have to play with the levels ahead of time to get a good general mix.

If you're not good with public speaking and/or making stuff up to say on the spot, prepare some notes about what you want to touch on. Maybe even run through the footage with your notes once or twice. Do this especially if you've already edited it to make sure you have time to say everything you want to.

You don't need to talk constantly either - give time for what you're saying to sink in and for people to fully appreciate the action.

Occasionally I've had issues keeping my accent consistent when delivering a voice over, especially if my prepared notes are quite extensive. Try imagining a specific person you're talking to that would balk if you started speaking in a different accent. Maybe they're sitting beside you and you're explaining the footage you just edited together to them. Keep it casual and your voice relaxed. Also, if you're doing a lot of footage at once, sip some room-temp water to keep your throat from going dry.

Tutorial Concerns is a very good point. Just doing the trick like a pro can be hard to follow sometimes. You can do it fast and awesome to get the 'wow' factor, but then consider doing it again at a slower pace or more of a step-by-step for the sake of the tutorial.

Opening and Closing

With editing, try to hook them early. If you're showing off a trick for example, try showing the trick or the cool result from it up-front. To pique their interest. To get them going 'how did you do that!?' THEN you show them how. If it's all set-up for minutes at the start of the video, they might not even bother waiting for the cool bit. With some types of content and footage this might not be appropriate or too cumbersome or come off as forced but, consider it.

This kind of structure is used all the time in professional movies and TV, where they might open with an action sequence (Skyfall) or something completely off the wall (Breaking Bad). Then settle back into a slower pace and build up again.

Also, keep your titles and credits short and out of the way. If it's over 10% of total runtime you probably need to cut it down. And no-one gives a damn that 4 different clans with their own 15 second intro movie participated in this 3 minute video: show us the goods first and then we'll decide whether we like your clan or not.

Endings are important too. Be careful not to ruin any good will you've generated with a bad ending. Common mistakes are being too whiney and demanding with requests to subscribe, check out other videos or to share and 'like.' It's not that you can't do those things, just be careful how you do.

That's enough, stop talking RC. :P

*I'm joking. At least I think I am...