One other habit I think we carry from our daily activities is the tendency to grasp. I see it in myself and in others: the instinct to grab onto uke's wrist or arm or shoulder or whatever, pretty much as soon as it's presented and comes into range --- even in waza that don't call for us to grab it at all. Even when the technique calls for gripping at some point, I think most of us tend to do it much too soon, never mind too hard. But once you've grabbed, it's really hard to extend, isn't it?
Our tendency to grasp? Oh, absolutely! Isn't that what human technology is based on? Grasping and gripping? The opposable thumb? We grasp, therefore we are. But now that we're not swinging in trees, are we trying to find some other outlet to satisfy that instinct, hence, overusing our grip?
Gripping hard essentially traps us up. You're right, it does make it hard to extend when making a hard committed grab. That's what I was saying. It also traps us and makes us vulnerable, that split second it takes to relax and release the grip could be enough of an opening for a counter. It's why I make myself use a relaxed and loose grip when being nage. Not flaccid, but relaxed. There's a difference in having just a slight amount of form or tension to hold the grip in place, but not overcommit to it.
On the other hand nikkyo wouldn't be nearly as much fun if uke didn't get a death grip on my gi.