Hard natal aspects of Sun/Pluto seem to have some sort of ego-destabilizing effect which can be exerted on the person himself or herself, or projected on others, or both done intermittently.
The square seems to evoke either a phobic or counterphobic response. With such square there is a sense of loss of control (which also implies personal disempowerment) and heightened vulnerability. Because the world keeps constantly evolving, people keep constantly acting and reacting, everything and everyone around is perceived as a potential threat that threatens to destabilize the Pluto-Sun square person, to knock them out of the very fragile sense of balance (of course, since this is a natal aspect, this is merely a person's own projection). The phobic response to this is to hide oneself, to stop interacting with the outside world that is experienced as self-shattering, to conceal oneself by becoming a very private person, or to wear a mask that isn't as threatening if it is destroyed. The counterphobic response is to go out and try to impose your own version of things, to dominate and disempower others before they have a chance of doing this to you. Most Sun-Pluto square people have both phobic and counterphobic situations in their lives, but there is usually a tendency towards either end.
In any case I think this leads to a rather rigid ego structure that attempts to stabilize (freeze) itself and is afraid of change as something that heralds it's disintegration. Any transformative forces are perceived as hostile to one's "I". This of course blocks any self-renewal and self-updating and complicates participation in the give-and-take of life - the person isn't changed by life much.
Way to revive an old thread!
I have this aspect (along with a square to moon and mercury on top of it) and I relate to everything everyone has said about Sun square Pluto:
1. It started early.
2. Severe mistrust.
3. Willfullness/inability to compromise.
4. Need to dominate others to feel secure.
5. Total transformation at some point by a complete sacrifice of the ego.
As a child, I especially couldn't tolerate being directed to do anything, at least not without feeling like I had some say in it. There was this strong fear that if I wasn't in control my world was ending. I spent a lot of time alone, I was verbally combative and mean to others. I was obsessive and demanding. My mother would always say to me "a tree that does not bend in the wind breaks".
My mother always thought my behavior was because my father "bullied" me when I was a baby. I'm told I used to hide under the bed and cry and scream when I had to go visit him, but I don't remember this. I do remember thinking very highly of him. He is very intelligent and sharp-tongued, and I was always afraid I'd be his target if I didn't measure up.
Pluto is about extremes, and I was totally knocked down and dragged through the mud before I changed. I had all these fears of not being in control even when there was no real threat, but it wasn't until a threat was real that I broke enough to be rebuilt. It was a threat on the life of my mother (moon/pluto square?). I never got hurt, but I couldn't protect her from the attacker myself, being a child. I felt utterly helpless, terrified, enraged and weak and it wasn't for my ego but for my mother. I got help and she was okay in the end, and that's all that matters.
A year after this event, I had none of these issues; I was quiet, withdrawn and considerate of others. I didn't care about being in control so much. I no longer had panic attacks over petty power struggles. My ego wasn't important anymore. There was still a lot of anger to deal with, and recovering, but now I'm pretty well adjusted. It was like I lived someone else's life.
It's a bit weird to understand how Sun square Pluto works after the transformation, since I feel like it's expressed very differently now.