I just finished programming a HTML5 game engine and is now trying to create content for it. I am quite unsure about how I would make the single player portion challenging and at the same time accessible.

Basically, the single player is a fixed scenario or storyline that the player plays through like an RPG. As usual, the protagonist overcome one difficulty after another, and eventually end up fighting the boss. I hope the difficulties and boss fight would be fun because they are challenging. On the other hand, less "hardcore" players might be put off because they get stuck or have to keep restarting.

There will not be an option to save the game at any time. This is partly because of the programming: I don't want masses of players hammering my database with save requests every few seconds. Partly because I think the ability to save the game at any time reduces the difficulty. So, I am using checkpoints or save points that players can restart from if they screw up. Perhaps at every checkpoint I will allow them to choose if they want to switch to an easier difficulty setting so they can get over a part that they're stuck with?

I was wondering if you guys have any thoughts about this? Have you guys come across any interesting techniques?

P.S.
I think Diablo 3's way of doing it (force everyone to run through normal mode before unlocking harder difficulties) is quite terrible.

Being an online game you have the advantage of gathering stats that help you tweaking the difficulty and the placement of the save points. You store where each player dies, and where he eventually quits the game. But also the time it takes to overcome difficulties and bossfights, if they are less than expected you can make them more difficult. (make sure you can disable stat gathering, if you fear masses of players hammering your database).

A technique that is used a lot these days (angry birds etc) is the score at the end of a level in the form of stars. Have some optional challenging extras that award the hardcore player exta stars, but allow the less hardcore players to continue to the next level. They might come back later to that same level to see if they can do better.