[Factus]

Below are two images. It initially started out as an attempt at a flat geometric background, but I felt as if it was too flat, so I added some drop shadows, and it looks less plain, but I can't explain what it is that puts me off about the shadows either. Do drop shadows completely ruin flat design or are there certain instances where they're needed? Pictures below.

With drop shadows:Spoiler (Click to View)



Without drop shadows:Spoiler (Click to View)

[SIM-2902786]

You need to adjust the drop shadow, that's all... You can make them luck nice you just need to tweek the settings more.

[LookerUpper]

I like it without the drop shadows.
Looks good.

edit: maybe try lowering the opacity of the drop shadows.

[Ayxi]

I like the 2nd one more, looks more pleasing to the eye.
The 1st picture is a little off for me, the drop shadows do not look good, I suggest lowering the opacity as LookerUpper said. Plus just fiddle around with them.

[Sharjah]

Second one is definitely a lot better. If you can't tweak the shadows yourself, then just keep it the way it is @ the second one

[Rayner]

Second one definitely looks better.

Depends if you're wanting a "flat design"

[✍ TʜᴇIɴsᴘɪʀᴇᴅOɴᴇ]

The real issue here is not adding or not shadows (although your shadows are too visible, try lower down the opacity). The real problem is the color contrast. Flat or not, if your color contrast is not bold enough, there it just won't look good. Try and play around with bolder colors.

[Gangin']

Instead of drop shadows you could try just a simple really thin black outline and mess with the opacity. Itll help with making the colors look a little bolder, but you may want to mess with the colors a little too as they are contrasing correctly