Golden Key
The golden key frequently found on various levels and often required to advance past a gate feel... very contrived.
It doesn't really add to a fun factor or grant any sense of accomplishment. Or increase difficulty.
I understand the intent and recall the days of playing Link to the Past, however the golden keys in -those- dungeons actually came as milestones. Perhaps a similar application could be used here. Perhaps the golden key might come as the result of a mini-boss? It isn't required to reach the end of the dungeon, but might turn into a section of notable chests (not the typical off-branch 5-chest dead-end)?
I remember there used to be hidden switches that you could find. If you activated them a golden key would pop up and you could use it to get some bonus chests. I thought that was awesome. I've been very disappointed to find that despite looking all over the place there seems to be no secrets anymore. No hidden switches, except ones that you are required to find in order to continue.
If keys are going to continue to be required I'd rather you replace all the golden ones with silver ones so we don't have to carry them around, drop them, fight some, pick them up, carry a little more, drop, fight, etc.

I'd like to see the Golden Keys used to activate machines and things instead of just gates.
Rotating levels that need a key and it changes the entire route you would take to the end.
That kind of stuff.
@Cire
I ran into one such dungeon early on, actually. I am pretty sure it was from the test dungeon in the main area of Haven. It was in a grassy level - there were blocks off the path that could be hit by a handgun. I remembered a similar setup in levels in the previous preview event, so I shot them, and lo and behold a golden key popped up on the path.
Haven't encountered anything like that in any of the arcades, though.
Also I like the idea of having to lug around the golden keys. Makes things more interesting and adds a nice touch to multiplayer when other players protect the key bearer.

Stone Garden has a hidden switch key, but that is the only stage I've seen with one.
The stages seem to be a little randomly shuffled. Sometimes the key will be far away from the door, including through several party buttons where the less than vigilant (me) can accidently lock the key behind you. Bringing the key through all that will net you some bonus chests.
Then sometimes the key room is just one enemy-less hallway away from the locked door. Anticlimatic and boring :|
I just chalk it up to the randomish putting together of set pieces.
Not sure if I was clear - I like the concept of the Golden Key, but its execution seems flawed due to the manner in which levels are generated "around" it.
I've seen it frequently that you find a golden key and the door it opens is ten just around the corner. No reason for that.
The type mouse and Cire have described where options present themselves is good.
I don't know that the lugging of golden keys actually does anything much - in any fight that matters the person carrying just drops it.
Yeah, I'd like to see more optional golden key material get worked into the level rotation.
I actually once came across a golden key that had a giant turtle guarding it during a solo run. While it was to a required section of the path, it was a nice challenge regardless, though I'd imagine it'd work better (especially in cases of optional key paths) if the key was being held by the turtle or something, rather than just being on a pedestal next to the turtle - some form of visual indication that to get the key you had to kill the big ol' bastard.
Alternately, gathering pieces of a key. That might mix it up a bit, although I can see how it might be obnoxious.
Basically just saying QFT here, but I wanted to break it out a bit. Usually, one of two things happens:
1) You get a key, walk uninterrupted, and open a gate.
2) You get a key, walk through some nastiness (spikes, mobs, etc.), and open a gate.
Usually the gate is necessary to pass through to get to the end of the level. Carrying the key is just a menial chore. As was said earlier, it feels very contrived.
There are some things I've seen that make it much more interesting:
- A well-guarded key
- An optional gate
- A gate that leads to a tough encounter
- A gate that holds some nice treasure
I want to be *excited* every time I see a key. I second the idea that golden keys should be harbingers of tough encounters.
I really like the way the keys are used in levels like Cravat Hall; there are two keys, and two doors. One leads to treasure, the other to the exit. The keys are protected by a gaggle of monsters.
So, the players have a choice: use the first key on the exit, or fight through twice as many monsters to get the treasure too.
I'd like to see more areas like this.