My biggest concern with the current system is that every single P800+ character has identical point distribution except in the core stats. This isn't a "maybe", it's an absolute.
Ways to alleviate this problem:
1. Eliminate round-robin distribution
3. Replace the 50-point hard caps with dimishing return soft caps.
I completely agree. These two points are some of my largest concerns.
1. If the developers are afraid that players are going to spend all their points in one or two places, then perhaps they can incorporate caps on the amount of points you can spend. This cap will increase as you in paragon level.
2. Yes, this will definitely be more desirable. That way you can still progress in the stats you prefer, albeit with less potency over time. That way it'll still feel like you're working towards the goal you want and not in the direction the game wants you to go in.
My second biggest concern is that a player never wants anything other than their main stat + VIT.
This is a big problem I have with the four attributes, it's that you only care about your main and vitality. I've never heard a player say "alright, I want to build intelligence on my barb to get more resist", no, they rather go straight for all resist.
No one cares to build outside what their class requires, and I feel that takes a bit away from decision making when building your gear, and it's going to have the same effect when you're building your paragon stats. I think adding another, smaller effect could help, like you said thorns on strength. It would also make less interesting stats like thorns more desirable, as you can then build a bit more effectively around them.
P.S. Don't know if it's been considered, but can health potion cooldown duration also be considered to the list of paragon stat points. It could make for an interesting choice. As I'm sure an "Alchemist" will be added to the game in the next expansion "wink wink".