There are a few significant problems that I feel need to be addressed, problems that may have contributed to Spiral Knights' lack of popularity as of late.
1. NORMAL DEFENSE: The major problem I see with normal defense is that against pure elemental, shadow, or piercing damage, its as if you aren't wearing anything at all. I feel that it should defend against all kinds of damage, but maybe half as much as the more type-specific defenses. This would make armor like the plate sets more effective in the late game and lockdown. (I mean come on, the plate armor de-buffs you by slowing attack speed, and then gives you the most useless defense in the game. That just isn't right.)
2. LOCKDOWN BALANCES: Lockdown is just a mess right now. You shouldn't have to conform to a specific meta to be able to play well. Either the Striker needs to be nerfed, or the Guardian needs to be buffed. While I'm not sure how you would exactly nerf the striker, I have a few ideas for the Guardian. Immunity to being staggered by enemy attacks, and a larger health buff (dependant on tier). Moreover, I feel that the dlc weapons shouldn't be allowed, but this is less necessary than the buffs and nerfs.
3. FORCED GRINDING: This is the one I have no idea how to fix, but I feel like it was one of the things that drives new players away.
Now I may be slightly biased because I play the Guardian Class (Both the Plate armor class and the Lockdown class) but when you sacrifice speed and style and get almost nothing in return... its not easy.
Normal defense is just a defense type like the others. If you have it, and get hit with an attack that is mostly something else, of course you're going to take a lot of damage. The same can be said for any of the others; I wouldn't expect Elemental to defend me from Shadow attacks. That's just the nature of the defense system: defend against what you're wearing protection for.
Plate armors already defend a decent amount more in the Normal department, but defense is very insignificant sometimes in general here. The difference isn't very tangible or noticeable in a way that would benefit the user.
Lockdown changes are an entirely different area and have been spoken of many times in the past. There are a lot of things that could be said and discussed, enough to where it could warrant its own topic rather than be combined in here.
That said, to build off of what you have, I would vote Strikers get at the very least a larger health penalty if nothing else. For Guardians, as I regularly run as one myself, I'd rather see the "no flinch" added to Plate lines instead when wearing the full set. Healing from the shield should scale depending on tier. Weapons are fine as-is to me.
As a side tangent that would benefit Guardians (and recons,) I'd also like to cast a vote for giving mist bombs "Moderate" status instead of "Minor." It makes immunity more difficult, but still possible.
For the grinding, that's the nature of pretty much any MMO. There needs to be something in place to slow progression way down and act as a barrier to both keep people from completing the game too quickly, as well as have that bit of incentive for people to buy energy to progress a bit faster. As I recall, the game throws you through the first few star levels fairly quickly before things start slowing down some.
Of course there are parts of this that are frustrating, and it may kill the experience for some, but that's just how it goes. It's different for everyone and people all have their own experiences with it.