hmmm. what you have is severe lack of contrast.
I know you have some kinks and flaws on the bricks but in real size the contrast is SO low, it's practically negligeable. The way you observed the diferent coloring of each brick is pretty clever an works pretty well, but you're being kinda shy on adding flaws to them, up the contrast on the shades a bit and dont use so many shades to transition from light to dark...it makes everything look flat in the end. Also, dont stop the variying the coloration on the the bricks behind the archs, it's a dead giveaway of your tiling...keep the variation on the bricks behind the archs, but not on the actual archs.
What is the setting? if it's urban you could add some graffiti....some plastered cement over them, or maybe have some of the bricks knocked off if it's some kind of oldage fortress....pick some kinda theme for the variation.
Before you do that, you need to pick a projection angle and a lightsource first. I asume your game is sidescroller since you chose a straight on sidescroller angle. I'd suggest some kinda slanted projection angle to show atleast a little bit of the side of the bricks, that will go a long way to add depth. check any beat-em up for example....sidescrollers have been using beat-em up perspective for ages now.
it's kinda hard to comment on the lighting (and everything else for that matter) when it's just these bricks out of context, depending on how high up on the screen they are you can make the arch part fade out into darkness....or change the lighting situation to bring them some kind of emphasis