I don't watch many Simtubers myself but between myself and my 12yo Simmer kid, I'll try to answer these and hopefully it'll help you.
1. Do you prefer speed-builds or "let's build"s, and why?
I have a preference for builds while my kid likes LPs more. For both of us, it's because of our personal play preference - while I enjoy live mode, I find myself drawn to building more. The kid is not very confident about her building abilities and she prefers live mode play (and challenges).
2. What bores you in Let's Play videos?
Unnecessary drama. One of my favourite LPs to watch is the Rags to Riches challenge, which already brings its own hardships. The other kind I like is pack-specific LPs. The things that I get easily bored with is typical soap opera style drama created by the player: cheating, relationship machinations, teen pregnancies, etc. Basically things that can be easily found in TV shows, soap operas especially, just to create complications to complicate a storyline.
Kid's comment: repetition of storylines in separate LPs or even within the same LP. Say, various Sim from various generations all having one-night-stands as a pattern.
3. What things are most likely to make you stop watching within the first few minutes?
Too much Alpha CC. I'm not a fan of CC in general but Alpha CC pulls me out of the Sims world almost instantly. (on a personal note, also the reference to MaxisMatch CC as "clay" because I find that derisive. I understand that it's the Simmer terminology, it just rubs me the wrong way.)
Also, completely irrationally, a player's voice. I have no explanation for this one, since it's entirely a "I won't know until I hear it" thing.
Kid's comment: lack of uniqueness. So if an LP is something that's been done over an over on other channels (like a Sim running off with Max Villareal ;))
4. Are you watching more for the game play or more for the personality of the player?
For me it's a little bit of both. A video can have game play I'm really interested in but if the player's personality doesn't sit right with me, I won't watch. On the other side, if I like a player, it doesn't mean I'll watch every video they've done. (Kid agrees on this one)
5. Do you prefer to see the player's face or just hear their voice? Or neither?
Kid: likes seeing face cam in LPs to see their reactions to the game. in builds, she prefers voiceover only.
I slightly prefer no face cams in either LPs or builds but I don't mind it, as long as the focus is more on the game.
(for a bonus, both of us like the commentary to be a mix of game-related stuff and personal anecdotes, as long as it's not only one or the other)
Hope the comments help you and good luck with your channel! :D