yes as PMB suggests... get more details... push the Wpoly a bit more 200 max... and add some epoly ( its common to post screenies with "developer 1 , r_speeds 1 " commandz in the console ) dont make everything so squre... add some slants or curves every now and again..
dont make a habbit of using "standard/valve" textures... theirs tons of free images and sites with texture packs...
the height and size of your map is VERY ideal for HL 1 engine... so toy around with it a bit more... maybe add a " unseen " ( for r speed reasonz ) 2nd floor... or maybe try adding " under ground tunnels/sewers " just to add more " environment " so its not just one level....
the water and a " bush corner 3rd pic " texture look wrong... im not sure how you rendered the water but try changing it up or get a better screenie