Damn, I bet threads of this kind already existed multiple times but I don't really care about it.
Anyways, what I know is, that many people design their levels by placing objects first and inserting sprites later. And I think that's horrible. In my opinion, leveldesign doesn't live from multiple paths (which are sometimes really annoying, confusing and/or unnecessary), also it doesn't just live from it's construction. But it lives from it's little sets of enemys, special objects and so on. What I mean is that I pay much more attention to small segments of the level themselves than to the whole level. And I don't enjoy hacks which don't pay attention to this at all.
For example, watch this video:
One level by Worldpeace125
You can cut this video into segments of 5 - 20 seconds length. And every of these segments will be a set at it's own with great enemy placement and interesting leveldesign. And that's why worldpeace is one of my favourite hackers actually.
When I start a level, I choose graphics and setting first. Then, I choose a set of sprites I'm going to use for this level. After that, I build the beginning of the level, with some coins and power ups. And then, I try to think of small, special situation around those sprites I have chosen. Not an island with a pit at the left side and a pit at the right side with one or more sprites on top. But a scene where the sprites and objects fit well together and are a nice challenge (like in the video I linked above). And when I built some of these segments and I run out of ideas for those, I build some more not-so-special segments (just some landmasses with sprites). And then I connect those in a way that it's not completely linear, but you still have to go through all of the segments (so you have to go back sometimes and such but you don't have multiple paths).
And when I play through hacks where all levels are designed like this (maybe some puzzle levels too, those are a nice touch), which aren't many at all, they are incredibly fun and give me a whole bunch of inspiration.
What do you think about this? And how do you design your levels?
Sorry for my bad english.