1. I've noticed some people get really discouraged when playing Defenders. Defenders get hit, get hit a lot, and get knocked unconscious a lot. A lot of players, especially those who are used to how 3E ran, just don't like getting beat on to the extent that 4E Defenders get abused. You really have to like being hit to play a Defender. Cautious players don't last as Defenders.
Yes, you have to like being hit. Yes, cautious and defender don't go togther. This is a given. It's like saying you need to like using magic to play a Wizard. Getting knocked unconscious a lot, I cannot agree with. Maybe it's playing a dwarf fighter, or something, but the defender tends to have good defenses, more HP, more surges, better surges, etc. I think that your defender should be using a lot more surges than anyone else, but if the defender is getting knocked out a lot, then I think people are treating the defender as an elite or solo. Defenders are tough, but are not designed to handle the encounter without support.
2. Passive players have real problems with certain classes. You have to take risks and put yourself in harms way for you to accomplish anything as a Rogue. Our most passive player previously preferred the Rogue above all other classes, and she just hasn't been doing well with the 4E Rogue at all. I've seen that passive Rogue players can't consistently achieve combat advantage, and that the Rogue class is powerless without it, while I've seen aggressive players with no regard for life and limb keep combat advantage applied 90%+ of the time. I've also noticed that the 4E Warlock lacks obvious power, but can equal the other characters by taking risks, provoking opportunity attacks to move and attack at point blank range, drawing enemy fire, and being a general pest. People playing Warlocks as being stand back and shoot characters have been very disappointed, while an insane Halfling Starlock who spent most of his time in melee range running around at full speed was often the most effective character we had. On the other hand, classes like Laser Clerics and Wizards are well used by passive players.
I can't speak to Warlocks, but our Rogue is very combat shy, as still really effective. She moves in and out of the combat fairly freely, and doesn't feel compelled to stay, or to stay next to a companion. She, unlike everyone else, runs into and out of the fights, rather than hunkering down to fight. She also will often shift/move into and out of total cover (like a doorway) and shoot, then shift back into total cover. She has VERY few healing surges, so her caution is well-founded, but it hasn't made her less effective. She just waits for her moment. However, she almost never just runs into a fight and stays there. If she's entering a fight, she has a plan for getting her little halfling butt right back out, preferably on her own turn.
It is true that Laser Clerics, Wizards, and Archery Rangers, and, I would say, sniper Rogues are suited to passive players. Some classes are suited to being passive, and others aren't.
3. Our loner player has a real problem with 4E. He isn't a team player, and he isn't getting the same results he got with 3E. In 3E, he was a powergamer who had all the big guns and dominated combat. In 4E, he doesn't synergize well with what everyone else is doing, and feels powerless.
Good.
A guy who wants to be a loner and dominate combat and have all of the big guns is a guy who wants to be better than everyone else at the table. If you want to satisfy him, let him have 5 more levels than everyone else. 4e is designed around balanced team play, with characters cooperating to fill roles. The entire system was designed to give the loner player a problem.
Seriously, either a "loner" player needs to play "alone", or he needs to realize he isn't a "loner" player. It's fine to roleplay someone who is aloof, or distant, or mysterious. To expect all of your friends to be satisfied playing second banana to your Mary Sue is not. Now, maybe you have this gaming group, but it's much easier to deliberately unbalance a balanced game than deliberately balance a game written to be unbalanced.