Wanting to paint something completely different from anything fantasy, I bought myself a couple of boxes of tactical marines and a dark angels veterans box because I like the models for a fun 500 pts force.
Here's the first tactical marine painted. I'll probably be adding the transfers on the shoulderpads when the unit is finished. Comments are appreciated ;) .
As I'm completely new to 40K I might have made some mistakes on the color choice of some parts, so bear with me :rofl

http://img177.imageshack.us/img177/3919/img0486us5.jpg

http://img514.imageshack.us/img514/504/img0487sb5.jpg

http://img187.imageshack.us/img187/4373/img0488tm3.jpg

http://img246.imageshack.us/img246/9453/img0489sn3.jpg

http://img337.imageshack.us/img337/9955/img0492kh4.jpg

Pretty good.
A few suggestions.
Thin your paints with water, but not just tap or bottled water. It has to be the tears of virgins.
Try highlighting a little more.
Other than that you have great potential..

The model looks a little dark, so you might try to highlight it a bit more than you have. I'm also not a huge fan of the bright green shoulder pad trim, but that's just me. Good work so far.

Ok, I'll add some more highlights ;Y

@xbanditsx : how would you paint the shoulderpad lining then?

The standard Dark Angels scheme has them being the same color as the rest of the model, so Dark Angels Green highlighted as normal. Again, you're more than free to paint them however you'd like, I was just saying that I wouldn't do it that way.

Nope, you did great. however my advice is: they'll look great held a foot away from you, but from 3-4 m they just look like dark green blobs. I'd go against what's previously said - no offence intended - and highlight more "pronounced" - search c'mon for examples
oh, and agree re water down paints.