House Rules of the Storyteller system in this game:

1. Characters should not be limited to a short list of Abilities.
Choose Secondary Abilities as you see fit. If you have access to the Book of Secrets or the Book of Shadows, you can choose any of the Abilities in those books for your character, too. If you don't, you can make up your own Abilities, with Storyteller approval.

2. Ability Rankings tell more than dice rolls.
Sometimes, in a pinch, you need the dice to tell you what's what. For every other time, trust the ability rating. Unnecessary dice rolls add nothing to suspense and a whole lot to time and overhead. We're avoiding them whenever possible.

3. Having a complementary skill can help you out on a tough check, but only if you have time to make use of it.
Sometimes a good Science skill really does help out with a Technology check. Using two skills to figure something out takes longer than using one skill, though, so you have to take the time to gain the advantage.
In game terms, you have to make a check with the complementary skill; successes on that roll reduces the difficulty of the other roll by -1 for each success over the first (-1 difficulty for two successes, -2 for three, and so forth), up to a maximum of -3.
This is covered under the Complementary Rolls sidebar in the Mage 20th rulebook.

4. 'Partial' success is success.
For twenty-some-odd years, White Wolf has been vacillating over whether you should count a single success on a roll as an actual success or as some kind of lame, gimpy 'Partial Success'.
In this game, it's a success.
Making one success does not mean you barely succeed, or that you look stupid succeeding, or that you succeed but should probably roll again next turn just to be sure. Any of those results would be success with a cost. Making a single success simply means you accomplish what you meant to with no frills nor extra benefits.

5. Failing a dice check usually doesn't mean your character does something stupid or even looks stupid.
If your character fails a dice check while he or she is attempting to do something that's obviously above his skill ranking, then he probably did something stupid. At all other times, failing a dice check means that something outside the character's control conspired against him that prevented him from succeeding in a task he rightfully should have looked awesome doing.
I mean, come on. Your character is obviously a badass--otherwise, why the heck are you playing him, and why are we following him? Badasses don't look stupid when they fail onscreen, they still look awesome. Maybe the other guy looked awesomer this time. Maybe the deck was stacked against him. Either way, we're going to avoid making characters look dumb just because we rolled crappy dice for them.
Choices make characters look stupid. Dice rolls do not make characters look stupid.

6. Failing a dice check doesn't necessarily mean your character doesn't succeed--it just means your character doesn't succeed in quite the way he or she would have liked...
Success with a cost is often an option. Sometimes the ST will offer a trade: success, but you do it loudly; or success, but you take longer than you'd expected; or something similar. Alternately, on a simple failure the player can offer one (1) trade along the same lines (so make it a fair one), which the ST can accept, refuse, or counter. These are options, and the player can always opt for failure instead.
Botches, on the other hand, are still botches, and never qualify for success at a cost.

7. Succeeding a dice check may not mean your character succeeds, necessarily--sometimes, the ST may offer a Complication on a skill check in exchange for an Experience Point.
When the ST presents a complication, he or she describes what's going to happen to the character instead of success. The details may be fuzzy, but it's not a mystery offer. Accepting the Complication means the character immediately gains an XP but fails the check (or has it rendered moot) due to circumstances outside his or her control. Again, this is an option, and the player can always opt for her rightful success instead.

8. Gaining XP is mostly about setting goals and seeing them through.
XP is all about growth, and the best way to grow is to undertake a task and then to complete it.

9. Demeanor isn't really a thing.
Demeanor is a non-binding game statistic the character doesn't get anything for and can change at will--therefore, what's the point? We're not gonna track Demeanor. A character has a Nature, and that governs how he regains Willpower, but he or she can demean himself however he wants without reporting it to the Storyteller.

10. Unless you’ve got a really good reason, every Mage PC starts with Arete 2, not 1.
Because who wants to play a mage limited to divination? Seriously?
I mean, do the folks at WW actually play starting characters at Arete 1?
On the other hand, most mages still don't start with an Arete above 3. If you want to start with 4, you'd better have a really good reason.

11. Regarding other supernatural critters in this setting, never assume you know what's what, just because you've played other WoD games.
I love tweaking, and I've done my fair share of it in this game to make other supernatural denizens of the night not quite what you might expect, if you know the rules.
Don't just assume the big, scary vampire is gonna run away with his tail between his legs just because you conjure an eternal flame. He might--but then again, he might not, and don't complain to me if he doesn't. It's my world. Vampires work my way.
Spoiler: Vampires don't twinkle in the sunlight.