Even if you are using the same monitor for different systems and that monitor is calibrated, each system can also have different monitor profiles. So that's one thing that can affect how your pictures look on OSX or Windows.
Another thing: the gamma of your system. By default, the gamma could be different on both system and it's possible the PC will always be darker than the default Mac one, it's usually the case. PC users usually have a 2.2 gamma and Mac 1.8. So if your goal is to look good ONLINE and 90% of your clients' computers are PC then you might want to adjust your gamma to 2.2 to have a better idea of what they see.
It's not uncommon for professionals to work with 2 monitor profiles; one for their print and one for web. In each profile you can adjust the gamma and white point (which should be D65.) This way if a picture needs to be re-calibrated for a specific audience online, you're more on target by using their monitor preferences than your "print" preferences. Since you are a photographer, that could be an option for you. That's what I do.
This link below gives more details and some tips about this. It should at least give you a starting point to research more on that topic:
http://www.kenstone.net/fcp_homepage/gamma_mac_pc.html
If you can check this for yourself, notice the green plants and skin
tones are way more saturated in Windows than in El Capitan.
It's hard to verify this because we all see your website's pictures in a different way. The best would be to show 2 pictures of the same website on PC and Mac next to each others! Even the screenshots get corrected.
I'm on a Mac, the green plants do look saturated when I use my "pc monitor" profile so I guess this has something to do with your issue. There's more details, shadows and mid tones when viewing them on the Mac profile I use. That's why I don't think the software color profile is the culprit, but the gamma or monitor profile could be.
Just a note on this: Your pictures still look great to me, as an amateur and not a professional photographer! But it's totally understandable that you can see what's wrong with them and you're annoyed by this.
Amazing answers! Thank you so much for all the help. I guess I will have to play with the gamma settings on my Mac. Again, thanks to everyone for the help!!! – None – 2015-11-02T06:29:28.910