A couple of things:
1) Bear damage in beta is currently too low. That is almost certainly the cause of any threat problems. It isn't anything you're doing wrong.
2) Tanks got gibbed for missing Shield Blocks back when bosses could perform crushing blows (and back when warriors were designed to be the only tank). Mechanics work very differently today. We want bears, and all tanks, to have more control over their own survivability because threat isn't really that hard to maintain once #1 is fixed. You still have emergency buttons for when things go wrong or when you know big attacks are going to land. However we didn't think a tanking model designed around using long cooldowns and standing around the rest of the time would be very fun. So the new design is that using your attacks skillfully will give you some mitigation, in this case Savage Defense. It's not often going to save your life and fumbling it isn't likely to get you killed, but if you play well, your healers may notice and say "Wow, you require less healing than many tanks I run with."
Think of it this way:
Beginner tank - can maintain threat. Probably 100% reliant on healers to stay alive.
Intermediate tank - can maintain threat. Skilled at using long cooldowns to live.
Advanced tank - can maintain threat. Skilled at using long cooldowns to live. Skilled at using short cooldowns to save healer mana and otherwise make things go smoother.
More skilled tanks also know more encounter specifics, such as when adds are coming, when to interrupt spells, when to move bosses and so forth.
Maul is a DPS alternative to Savage Defense, and is comparable to Heroic Strike. You shouldn't need Maul to maintain threat (which as I said above may not be the case on beta right now, but that's the intent).
In what situations would you want DPS instead of mitigation? There are several: Maybe you're an off tank and taking little damage. Maybe you want to solo as Guardian. Maybe you're running a dungeon you out-gear. Maybe a new add just arrived, and it's important to get one swing in before it heads to the healer, assuming you can live without SD for a few seconds. (In that example, you're not spamming Maul to fight to maintain threat; you're just establishing initial aggro, which we still want in the game.)
To be honest, we considered pulling Heroic Strike and Maul from tanks, just because we thought they might be a trap. But we decided there were enough situations where more DPS might be desirable. To use my previous examples, inexperienced tank will use Maul too much. Intermediate tanks might forsake Maul altogether. But expert tanks will know when they should use it.
You can try to keep up Savage Defense as much as you can on challenging content, and the best tanks will, but I would still consider when to use it. I would not save it for a big boss smash that isn't going to land for 20 more seconds (because chances are you can use it now and then later), but if one was going to land in 8 seconds, it might be worth holding Savage Defense for then.