It IS easy! Get a trial download of Xara Designer or Xara Photo & Graphic Designer (same program, without website-making stuff) and mess around with it. I used this program for years at work, before I retired (multimedia designer and illustrator). The Xara photo-editing tips at
Xara Users • Login include other easy ways to make colored filter effects, graduated ND filters... so much easier to do after you take the photo instead of during it.
Only exception -- the polarizer. Yes, you can simulate polarized sky or knocked out reflections in Xara or Photoshop -- but it's very hard to do correctly, and takes a VERY long time. Much better on the camera... particularly for reflections.
Re HDR... it is exciting when it works, and it works if the viewer never notices that you are doing it. Most HDR images are really horrible! I use a small program called Dynamic Photo HDR 5 from Mediachance. But there are other ways to go at it, like Photoshop's Dodge and Burn tools, or using Xara to copy an image on top of itself, and then cut away part of the top one to reveal a different-toned one below.
Graphics techniques are endless. And with a lot of effort, you CAN turn that sow's ear into a silk purse. But it's so much easier to start with a really strong clear image. Which gets back to why I like film... you take so much fewer pictures, and you think a lot more about each one, and so you end up with images that need a lot less post-processing...