For myself it was that hands were important in guard retention.
I always was reading stuff about rickson and amasing blackbelts being able to keep the guards without hands...so i was thinking that you did not really need hands to keep the guard....and i lost a lot of times trying to play this game before understanding that to keep the guard you need to control the hands or body or legs of your opponent with your own hand.
Dont get me wrong....i was using my hand to push but i was not really practising the gripping part.
Ps : it was in 1998 - 2002 so we did not have access to all the information we have nowaday.
And you? What took you a long time to understand?
At 36 i am going back to bjj so maybe this thread can open my eyes on some point and makes me gain time. 
Probably the opposite for me as an example.
Injured my shoulder and was rolling with legs only.
It really emphasized just how much you should work with your legs to retain guard, and just how useful they can be when you are active with them.
So many other little things that just click after a while too.
Not being flat on your back
Lines of defense and offense (feet, knees, hips, hands, elbows)
Chaining attacks and baiting
And about a million other things that I always forget!
But in terms of hands, grip breaking before trying to pass guard, or not letting them establish grips first. Lot's of hand fighting before passing/takedowns/sweeps etc.
Learning to enjoy the experience of training and the grind itself 
That fundamentals are still where it's at. Got caught up in trying to learn rubber guard and lots of low percentage/flashy subs when I first started. Wish I had spent more time learning to wrestle, working on high percentage subs, good base, guard passing and guard retention, and learning how to chain together attacks. Once I started doing those things my game took off. And then it was actually much easier to add in the flashy subs and funky guards because I knew why and when they worked.
Grip priorities and hip movement.
Hip movement
It's just man wrestling.
50/50 in the gi. Lawd, yes. I just ranted about it at the end of this:
A more advanced BJJ tutorial on getting out of and taking advantage of 50/50 position!
It took me a long time to begin to understand the role that strength plays in technique.
djkingcrab - That fundamentals are still where it's at. Got caught up in trying to learn rubber guard and lots of low percentage/flashy subs when I first started. Wish I had spent more time learning to wrestle, working on high percentage subs, good base, guard passing and guard retention, and learning how to chain together attacks. Once I started doing those things my game took off. And then it was actually much easier to add in the flashy subs and funky guards because I knew why and when they worked.
That sounds very similar to Stupidnewbies thread on his bjj journey. It was a good read.
Josh Vogel - It took me a long time to begin to understand the role that strength plays in technique.
To quote mark coleman "the strength is the technique" which is bullshit considering how goood a technical wrestler he was
asdf -That there is a lot of low percentage bjj taught, especially back when I started. And that I should not spend time on these low percentage moves.
By low percentage, I mean stuff that only works on white belts or people you out-strength, or only works when the opponent is unfamiliar with the move
True.
There is the jiujitsu people pretend exists and then there is the jiu jitsu which actually exists in live rolling. The latter is a much smaller than the former. There are patterns which occur over and over and over again in live rolling, while other patterns --which could happen-- almost never occur. There is a reality to live rolling which is inexplicably different than that which the theoretical practice of jiujitsu prepares one for.
Also, there is a lot of stuff I think BBs do when rolling with their own students --where they can dominate every moment of and element of a move. THEN, they teach it at a seminar or post it because it's new: "Hey guys, here's a new move I've been experimenting with...".
But when you get right down to it, it's not the the kind of thing they would have much success doing against someone around their same skill level. Often, it's a move that requires you essentially be much better than the person you are rolling with.
The BJJ world is stuffed to the gills with this kind of material; technique that is awesome in THEORY and actually does work well IF you are way better than your opponent.
A lot of newer students can't really tell the difference. The constant push for "newness" in BJJ --while sometimes leading to great innovation-- many times just leads to a lot of technique without depth.
Outstanding post shen! So much truth
The importance of always keeping your opponent off balance when playing guard and not allowing your opponent to "settle in" when they start working to pass.
Also the importance of grip fighting. I used to reach a point where I would get bored of fighting off an opponent's grips either while passing or playing guard and just going for it, but now I will continue to break grip/make grip the entire round if necessary.
juanderlei - The importance of always keeping your opponent off balance when playing guard and not allowing your opponent to "settle in" when they start working to pass.Yep....grip figting is one of the big keys.
Also the importance of grip fighting. I used to reach a point where I would get bored of fighting off an opponent's grips either while passing or playing guard and just going for it, but now I will continue to break grip/make grip the entire round if necessary.
I would buy a DVD about grip fighting
Attack of Knee line and hip line are the key to passing
shen -So true. I discard WAY more stuff then I keep.asdf -That there is a lot of low percentage bjj taught, especially back when I started. And that I should not spend time on these low percentage moves.
By low percentage, I mean stuff that only works on white belts or people you out-strength, or only works when the opponent is unfamiliar with the move
True.
There is the jiujitsu people pretend exists and then there is the jiu jitsu which actually exists in live rolling. The latter is a much smaller than the former. There are patterns which occur over and over and over again in live rolling, while other patterns --which could happen-- almost never occur. There is a reality to live rolling which is inexplicably different than that which the theoretical practice of jiujitsu prepares one for.
Also, there is a lot of stuff I think BBs do when rolling with their own students --where they can dominate every moment of and element of a move. THEN, they teach it at a seminar or post it because it's new: "Hey guys, here's a new move I've been experimenting with...".
But when you get right down to it, it's not the the kind of thing they would have much success doing against someone around their same skill level. Often, it's a move that requires you essentially be much better than the person you are rolling with.
The BJJ world is stuffed to the gills with this kind of material; technique that is awesome in THEORY and actually does work well IF you are way better than your opponent.
A lot of newer students can't really tell the difference. The constant push for "newness" in BJJ --while sometimes leading to great innovation-- many times just leads to a lot of technique without depth.
Learn to say no a lot to get better.
Took me way long to learn top position. In fact I got my blue belt and I was still having trouble holding it against bigger guys. No problems now but still improving.
shen -But that's also how u get good at new things. While today it might only work on white belts, the goal is to eventually add it to your game for all levels. Not saying this is for all moves, but the concept is good.asdf -That there is a lot of low percentage bjj taught, especially back when I started. And that I should not spend time on these low percentage moves.
By low percentage, I mean stuff that only works on white belts or people you out-strength, or only works when the opponent is unfamiliar with the move
True.
There is the jiujitsu people pretend exists and then there is the jiu jitsu which actually exists in live rolling. The latter is a much smaller than the former. There are patterns which occur over and over and over again in live rolling, while other patterns --which could happen-- almost never occur. There is a reality to live rolling which is inexplicably different than that which the theoretical practice of jiujitsu prepares one for.
Also, there is a lot of stuff I think BBs do when rolling with their own students --where they can dominate every moment of and element of a move. THEN, they teach it at a seminar or post it because it's new: "Hey guys, here's a new move I've been experimenting with...".
But when you get right down to it, it's not the the kind of thing they would have much success doing against someone around their same skill level. Often, it's a move that requires you essentially be much better than the person you are rolling with.
The BJJ world is stuffed to the gills with this kind of material; technique that is awesome in THEORY and actually does work well IF you are way better than your opponent.
A lot of newer students can't really tell the difference. The constant push for "newness" in BJJ --while sometimes leading to great innovation-- many times just leads to a lot of technique without depth.