Get rid of the two landscapes.  The last two look like they have a giant sensor spot on the right side.  Try another landscape in place of the ones you have maybe a better version of the cityscape.

In general, the images look like you indeed did a lot of photography, so you more or less know what you are doing. However, stock requires a bit different thinking, images don't have to be "great to look at" but useful and usable. The images you show look technically sound, though you will always find little things to improve. But the usability is limited in several ways:

You should avoid cropping elements out of the frame:
- In the third image (guy listening to music), the left shoulder is cropped while there is enough (too much) white space on the other side
- In the city night shot the top of the skyscraper in the center is cropped out (also the horizon is tilted)
- In the last shot the guy on the telephone is cropped as well

You need to learn to clean up images - there is lots of dust on the black suit and the graduate.

In general, "bright" is more useful for advertising than "dark". The business man would be more usable on a lighter background, like a rows of windows or white walls in the back.

Why to apply to istock in the first place 
The people shots are quite okay. The cable shot could be quite useful as stock. The Tiger looks okay too but it also looks like a picture from a zoo, if so you are aware that you need a property release for that in most cases?
The nightshot of the city is very bad, bad framing (you did cut the roof of the building) and the horizon is not straight. The landscape is very boring and the horizon doesn't look straight to me too.