Some time ago, I was thinking about a full PvE Crusader (Hi Spillion ), and some thoughts I would like to share:
Synergy: It would be great for us if we could have 2 talents on the same build without losing the full def/holiness tree: One gives 10% DEF/DMG against Bosses, and another one gives you aggro boost (Sorry, I forgot the talent names, i don't have the game installed anymore). It's pretty sad that you can't have both, and if you chose the 10%DMG/DEF, you can't keep aggro long enough.
Empathy: You can't heal friends because you start taking true damage from it (ignores your defense), there is already a topic speaking about it.
Early-levels energy sustain: When you hit 60 and get your Runic Hammer+Wild Boar card, you pretty much don't need energy pots thanks to the infinite energy sustain/management from talents (-20% cost while in combat, 5 attacks = 1 free spell), but that only happens when you have gears to fill the need. Crusader is energy-thirsty and at early levels that could be improved (maybe with a scaling-cost system like the Leap skill).
Not much into speaking about its tankiness, most people ignore the racial/elemental resistances but they don't think about the percent damage absorb it gives. After 50k def you can pretty much go full racial/elemental resist because def stop scaling (percent-wise), yet most of the people just spam DEF till 180k+ and keeps thinking that Crusader should be a better tank.
I think that Crusader is a pretty solid class (pun intended), but it doesn't have a good synergy which makes its gameplay boring and tiring. Plus, you can't really FARM with it because it lacks damage and if you don't have "decent" gears, you can't do even your job as a tank. After some time playing alone, most of the people just give up and create a Sorcerer, since they can farm alone with far lesser effort gear-wise. You can't have a PVP-only class or a Boss-only class, unless you want an ALT-BOT to specific tasks, which is game-breaking for those who wants to play as Crusader.
Thanks for reading, nice topic too.