You really don't want to see what happens to Counter-Attack when you disallow reactions during your own turn. A bad guy with counter attack and a nasty weapon that can still parry like a lightning claw will get you into melee and effectively beg you to attack him, knowing that he can avoid your attacks reliably and turn them into a counter attack that annihilate you, you can step back and get swatted for free and be unable to avoid it, you can disengage and waste your whole turn, or you can attempt an acrobatic disengage and might have the ability to do something for that turn if you're Slaaneshi. Suddenly one talent becomes really dickish action denial.
Worse, a player with counter attack who knows that baddies cannot avoid counter attacks will get into melees where he can pin down a much nastier foe in the same fashion as above. I've seen boss fights become tremendously one-sided as a result of this wording. We ended up having to rule that your reaction(s) refresh at the START of your turn, and a pair of melee monster characters would often swing, counter, counter, dodge in a chain during one turn to blow each other's reactions but without breaking the rest of the combat.
As for the Overheats trait, it generally ensures that nobody uses plasma weapons, especially since getting best quality ones is so difficult. A couple players used the purified 'cool burning' plasma from one of the Rogue Trader books (probably Into the Storm) that removed the overheats trait but reduced damage a little bit, since that made more sense than the Black Crusade version that has no drawbacks (why isn't this available to imperials?).
We've also found plasma and melta weapons to be rather rubbish as written and added +1d10 damage to all of them, retroactively added the melta rule to all melta weapons from any book, and retroactively added Maximal mode to the majority of plasma weapons from older books as well (unless it's listed as a civilian model that is incapable or something).
I never understood why a plasma weapon could overheat on the first shot of a combat when it wasn't a poor quality weapon. If firing it builds up heat, that would make sense, but otherwise the term 'overheats' for a weapon that has a 10% chance of blowing up regardless of whether it's been heating up from continuous firing doesn't fit, that sounds more like the trait should be called 'poor containment' or some such. I might rewrite the wording of the Overheats quality to reflect that.
Now, I completely agree that plasma weapons should be less reliable for chaos - the Imperium still has Forge Worlds like Ryza that hold the secrets of holy plasma (even if their knowledge has declined somewhat since the days of 1st edition when plasma weapons didn't overheat at all), but the heretics have to make do with salvage and poorly understood schematics. It's the same reason that heretic power armour has only a portion (3) of the subsystems working, even with a Heretek repairing it for you (5 qualities instead of 3). By this logic, it makes sense that it would be a full hit rather than a 0 Pen hit since the warp plays havoc with delicate systems and venting is often substandard. However, since we're already adding 1d10 to the damage of all plasma weapons at our table, that becomes largely unnecessary.
I would rule that a field would be able to block the damage from a plasma containment failure IF it could normally block melee attacks, since the attack is coming from melee range. I would also allow a player to dodge (fluffed as dropping the weapon) if it overheated to avoid the damage - that's one less reaction to avoid incoming enemy fire.
Edit: Why the hell aren't paragraph breaks working? I spaced that out, but it still posted as a wall of text
Edit #2: Ah, no-script on firefox breaks the posting tools and takes your paragraph breaks out. <sigh>