I do love Visio and I find it quite intuitive to use (the later versions not so intuitive as the others (i.e. those from 2004 and earlier) but it is too darned expensive for a retired guy to consider! I used it a lot (not at the high level you apparently do) and found it very handy for generating (mainly) drawings. Most of these drawings were incorporated in Word documents and used to both explain and document my work. Very valuable. BUT ... expensive. SmartDraw may be better BUT ... it is still expensive. Sigh.
Another free image handling product I love is FastStone Image Viewer. It does far more than viewing, while being very simple and easy to work with. FastStone.org also offers a free image resizer, great for batch processing. Features complex renaming, resizing by percentage, to fixed pixel dimensions, fixed dimension on one axis, and more.
Go with Paint.NET, you won't be sorry. I've been using it since the 2nd or 3rd release as a Photoshop alternative at work and it's dead simple. From the functions you mention you need to a few selection tricks that are way more difficult in Photoshop, IMHO, it is a great piece of software. For web content, its best feature is taking an existing image and using the color picker to get the RBG code to use in your HTML/CSS markup. For heavy photo editing, I would not recommed it, but it does handle the basics like brightness, contrast, levels, sharpening, etc., well enough that you may not care. If you need something more powerful, don't forget that Photoshop Elements has most of the power of regular Photoshop but at a 1/5 of the cost.
We switched over to SmartDraw at work a few years ago. Overall, I've been unhappy with it and wish we would go back to Visio. I agree that automation can be a killer for a product. There is much automation in SmartDraw, but you don't have to use most of it, which is a good thing. But when you must use it, you get inconsistent results.
I mainly make flow charts. Two of the things I really like in Visio is having the text/shape resize to fit the contents while typing and how you can exactly line up connecting lines. I have SmartDraw 2008 and it fails miserably for both of these.
While it can resize fonts/shapes, it does so inconsistently. For example, sometimes all of the lines of text shrink to fit; sometimes just 1 or 2 lines shrink, but the other lines stay the original size. Worse, once you "get it right" and then save, close, and reopen the document the next day you'll find some of those fonts have been resized again and may or may not look like when you saved!
A very similar thing happens with connecting lines. Sometimes you can line them up, sometimes they're off a pixel or two. When you print it, it makes for a very unprofessional looking document when they're off a bit. And when you save, close, and reopen, the lines sometimes shift "automatically" on you.
And that's all if you can get the lines to connect right in the first place. The way SmartDraw makes you right click to change a connecting line from a right-angle connector to a "bracket" connector is very un-user-friendly. (Visio is much better in that if you want to have an extra "bend" in the line, you click and drag the little square in the middle of the line.) But worse again, is that SmartDraw will sometimes automatically change your line style to what it thinks it should be, even if you manually change the style.
I have other rants about SmartDraw, so I hope you have better luck with the "VP" version that's out now -- maybe they've made significant improvements since 2008.
I've been using StarOffice Draw (now OpenOffice Draw) for a couple years as a replacement for Visio. It allows you to create components and save them. I've created many standard electronic components that I use regularly. I don't use Visio enough to justify the cost.
My problem is that I spend so much of my time creating diagrams -- and I've spent so many years honing my skills with Visio -- and I already have Visio licenses for all of my computers -- that it would be very difficult for me to justify migrating to another tool.
I tried the SmartDraw demo (after using Visio since version 4). SmartDraw may be good for some, but it is somewhat limiting ... I too, use the "Fragment", "Union" and other functions in Visio. If you need to do any "cad like" drawings ... Visio is much better. I also have a large library of Visio drawings (and shapes) that I can and do re-use.