Once of the more difficult aspects to get right in a game is the difficulty curve. Start the game off too easy and the player loses intrest, start it off too hard and the player gets frustrated. Same thing happens if you ramp up too slowly, or too quickly. The trick is to slowly introduce new elements to the player as the level progresses, requiring them to think before the act, and to be more cautious.
Let's work on an example. Here's a list of gameplay elements:
Now, throw all of those things in the first level, and your player is overwhelmed. Let's say we've got a game that have five levels. Five levels, five hazards, five abilities. Uh oh! I see a pattern :D Introduce the player to simple elements first, like the bad guys. He can jump on their heads and kill them a la Mario Bros. So great, level 1 we introduce the player to Jumping and Bad Guys. Level two, the Jumping and bad guys are back, but they're mixed in with Pits and Ducking (Ducking to avoid the new FLYING bad guys, of course ;) ). Level three, we still have everything else, but now we introduce Lava and Double Jumping. This continues until the last level where we have all five hazards and all five abilities.
It's important to remember to balance the hazards in a level as well. Let's look at level 5 in our example above. There's a lot going on there that requires the player to respond in different ways. If he's trying to jump over a pit, while dodging falling rocks that are coated with Lava and bristling with turrets that are shooting bad guys... he's going to get overwhelmed and the game stops being fun and starts being a chore. Remember that each hazard is not a whole in and of itself, it is a part of a greater whole. I like to look at it like the level as HAZARDS. One set of Hazards. Each individual hazard works as a fraction of that hazard. So 2 fifths of the hazards are bad guys, another fifth is pits and lava, gun turrets make another another fifth, and the rocks make up the last. So mostly, you're fighting off baddies, but occasionally you run into pits and lava, rocks, and gun turrets, just to make your life a bit more difficult. This let's the player deal with one or two hazards at once, rather than all of them at the same time.
Start of by plotting out the easiest level, then plot out the hardest. From there, create a nice difficulty curve between the two. And don't be afraid to cut hazards and abilities. It may be the most awesome thing in the world to have an ability where you character can swoop down from on high, spewing hot molten lava from his bum, but if there's no place for it in the game, then it has to go. Likewise, it may seem vitally important to include a bone crushing baddy that fills half the screen... but if it doesn't fit, it doesn't go. Keep your list of features as short and concise as possible, that will help you to create a good level arc, and also help you ensure that the features you do keep are solid and polished.
Talk to you again soon!