i think there are a few thing you can do. the attika for example looks too strong. try to compare with photos of a temple to find the right proportion for it. the columns have normaly less (and thicker) slices than in your image and they are not visible so sharp. keep it up!

Taurus66 is correct, the columns are not really correct: the Greeks created the columns starting slightly larger at the base, tapering up a little to the capitals. The triangular pediment of the roof facade is a little too imposing, it appears too heavy, that the columns couldn't support it. The texture is a little dark as well, and wouldn't cover the whole structure. Remember, each stone would be showing and the texture would only wrap on that individual stone, not all over the structure as one surface texture as you have rendered. Plus the Dentals (the square blocks of stone just above the column capitals) are way too large. They really should only be about a quarter the size. (that's why they are called Dentals, because the Greeks thought the detail looked like teeth) The lighting is okay, shows a bright sunny day, but the sky map doesn't match. If there is such hard shadows, this would normally denote no clouds at all. To repeat Taurus' comment, you should study images of the structure. In the "rules of art" it is known as: "Attention to Detail". Research, research, research.....

Thanks for your comments! I started all over again, because I thought it took much more time to erase all faults in this picture than to make a new one. I've decided to take the parthenon as example, so I had a good possibility to model all the proportions more exactly. I almost finished modeling by now, hope I'm done end of the week to be able to present the new one ;-)