Grid + set the outer border of the unit box to light up blue when someone is below 70% mana. Works perfectly.

You can set it so show any category of debuff as either the icon of the debuff itself over the middle of the player's frame, or you can simply set it so show a color coded dot in a corner of the frame based on the debuff type (Poison, magic, etc). You can also set it to filter and only show specific debuffs by name, category, or just about anything. It is almost infinitely customizable if you want to put in the time. I suggest browsing the 2.1 UI thread for many examples of Grid in action, configured many different ways.

There's also a plug-in for Grid that will show mana bars available from files.wowace.com.

GridManaBars

It was slightly buggy last night though when I was testing it. It works fine if you show vertical mana bars, but when switched to horizontal mana bars, it didn't always show them for everyone.

Yea, I tried that plugin out, but I think I have my grid frame small enough that adding a mana bar to the same area just made it nigh impossible to accurately pick out HP on players along with all the other stuff that is already in there like debuff icons/etc. It does work fine, and if I used a different layout might be better, but for me sticking with a simple blue dot in a corner or a blue border to a frame to indicate people that would get full use of a mana tide is all I really need to see.

You can set it to hide non-mana bars through the options.

After having seen what stopcasting could do for offensive casters, I decided to test it with healing. However, I'm having trouble getting the exact functionality that I want.

When using Clique with a macro (/stopcasting /cast Holy Light Rank 8) I lose the ability to click the bind on any unit frame and have the heal go to that person. Instead, it goes to my current target. Is there any way to fix this, or will I have to bind /stopcasting to some other button? It seems really awkward to have to use 2 buttons, and I think it would likely negate most of the gains from stopcasting since my lag isn't terrible in the first place.

Make your add to your second line so it looks like this:

/cast [target=mouseover] Holy Light Rank 8

Problem solved.