I'm not a visual designer but heres what I think is making it look so bad. The task bar is way to thick. The flat design of the icons needs some shadows or something to make them pop a bit. The rounding on the corners is way to much and needes to be dialed back significantly. There is both a top bar and a bottom bar, pick one ffs dont waste screenspace with both. Also top bar is transparent while bottom is fat and grey. I'd keep the top bar and scrap the bottom.

As far as OS's that look good, #1 KDE Plasma with breeze, MacOS snow leopard, Windows xp and vista.