I think you shouldn't worry much about gameplay, because what I've seen from your last video, your game already is great from that respect.

You should make a difference between
- gameplay - that's mainly about the control and feeling of the movement and interaction,
- level design - that's mainly about the challenges and directing and tasks within the levels,
- grafics - that's mainly the visual part of that all.
But, most probably it is easier to discuss those things without making these differences, because a lot of people in these forums find it easier to mix these things up, although they are 3 totally different tasks within game production.
You should consider 'story' and 'character design' as 2 additional important tasks.
To give an example of a sidescroller that I liked most, because gameplay, level design, grafics, story and character design are fantastic:
'Heart Of Darkness'.

Sorry, maybe this isn't much of help, because 'Heart Of Darkness' is the product of a big skilled team, and it is no chance that a lone wolf can copy it.

BTW: 'And Yet It Moves' is a good example of an interesting lone wolf product.

I think the main challenge to finish your game is finding a unique and easy to accomplish style.

Don't worry about what makes your game better than others, care to make it good and unique - and as delightful as possible.