Aside from the strength bonus, one to two points can make all the difference in a meat grinder game where people die in 2-3 hits.  The unfortunate thing is that most weapons that go through that kind of armor without problem also tend to roll a ridiculous ammount of damage along with it. 

Weapons with a high Penetration don't always have high Dam, so such weapons can counter power armour without unduly screwing over the other players. Look at the hellgun (one of the standard options on combat servitors) for one example.

…huh, never noticed that.  Back in DH they only had a pen of 3.  Guess they decided to make a point to using one.

Adeptus-B said:

Black Crusade addressed this by encouraging that you add one or two NPCs that are kitted out to deal with Marines that focus their attention ON the marines. If you've got ten soldiers charging the players, add one soldier with a hotshot lasgun at the back of the group that takes pot-shots at the power-armoured player.

Errant said:

Smart target selection also considers a target's offensive punch. A guy in power armour packing a lasgun is generally a lower priority target than a bare-chested guy blasting away with a heavy bolter. If your power armour guy doesn't pack the same offensive punch as the rest of the group then he's somewhat more survivable, but not necessarily overpowered.

Bowoodstock said:

It seems to be, in large part, a response to the update in the Imperial Guard codex between the two, and the move from AP 5 to AP 3.

A good question might be, what are the other players, and what are they sporting in the Koronus fashions? Power Armor is rather rare, and can be rather effective, but Storm Trooper Carapace is pretty close, without all the extras, and could make the rest of the party feel safe. Force Shields are one of my FAVORITE pieces of gear, if you have Into the Storm, and RT groups are among the few who can "easily" acquire them. They aren't quite armor, but even the Refractor Field can be very efficient, can allow your character to "look a certain way" that isn't armor-monger, and isn't likely to be checked at the door.

Otherwise, yeah, remind him that the suit has drawbacks, making him easier to HIT, and easier to find. Power Armor is a great thing, but it still has to balance with everything else.

Errant said:

When the BC Forum first started, I asked how you keep the adversaries specifically kitted out to challenge CSMs from auto-killing the Mortal PCs when their line-of-sight to the CSM gets blocked in the course of combat, or if the Horde charging a CSM ends up close enough to also attack a Mortal; I was told that these circumstances never, ever occured. This boggled my mind, and it took me a while to figure it out: all of the playtesters use narative-only combat, rather than miniatures like I do. With minis, there is no way to 'fudge' things to keep the heavy-hitters focused exclusively on the CSMs, but using narative-only, 'line-of-sight' and 'close proximity' can be whatever the GM says they are. Something to keep in mind if you are considering persuing the 'two levels of adversaries' option.

To be frank I don't rate this game's damage mechanics. However, one aspect that I don't see mentioned much is cover.

If people are in a firefight they should seek cover. Only the locations sticking out should be vulnerable to most weapons. The mechanics are there and it is up to the GM to use them - by having the NPCs use cover intelligently. NPC snipers dug in behind rockcrete should be able to mess up even a power armoured PC.

 I've played as a Character in Power armor before and as a GM having to deal with it. Frag rockets will mess up his day, the raw damage they put out is killer, and it builds up quickly. Krak, melta, and plasma will ignore most if not all of his armor.

But if you're looking for something to discourage him consider that it takes a space marine, with all their attendants hours to put on the armor and that in a full suite, in battle filed conditions, giving him any kind of medical treatment will be next to impossible. Power Armor is also insanely complicated to take off so he would need a tech priest and a good bit of time to take it off even out of combat.

Something else that you could do is send psykers against him. Some psychic powers, such as Psychic Scream and Soul Killer ignore armor, so you wouldn't be flat out killing the other characters, but the guy with power armor is no longer undefeatable. Also, other effective powers would be the Dominate tree. Compel, Dominate, and Puppet Master will basically turn the walking tank into a formidable enemy for the rest of your players. You could also have a Biomancer using powers such as Inflict Pain, Agony, and Blood Boil, to make the Power Armor player completely useless

I don't find a problem with some players being much stronger than other players in a fire fight. The game is much more than who can deal or take the most damage in combat.

As a GM I try hard to come up with adventures (or modify purchased adventures) to enable players to get by with clever tricks, social interactions and other stuff. There will of course be fire fights, but if there is enough variety even the other players will enjoy it when their super tank gets to work and they can huddle in behind him and feel protected. A power armor will be awkward in social situations, and if he refuses to leave the armor behind, he must simply sit out of some of the content.

Also make carapace armor and best quality xeno available. That only puts players a few points behind, that is not terrible much. Maybe let the commanding rogue trader get a 30/5 shield or something similar. I have a player with light armor in my campaign that insists on wearing light armor, to prevent him from falling too far behind the power curve I actually let him get some shielding tech into his armor as a reward for his good service to the empire.

I'm going to assume that the player in question isn't a Astartes sargeant, so his breathing is self-contained with a helmet. If not, however, then poison gas and viral grenades would molest him nicely. Maybe you can arrange for him to "lose" his helmet? Leaving it behind somewhere? He's got to drink or eat sometime. Furthermore, a power-armored opponent is a huge threat and should, thus, be neutralized first; Have all enemies concentrate attacks on him, especially heavy weapons. If they are aware that he is using power armor, Explorers often become famous, then it is almost guaranteed they are not going to challenge the explorer without heavy weapons. This would also increase the survivability of the other explorers, who may even find themselves preoccupied with saving his life if he is constantly wading into battle, against enemies who are intelligent enough for tactics.

But a "ally" in power armor might also be a significant threat to the party. Imagine the blight for the explorers if he were successfully attacked with hallucinatory weapons; gas grenades, poison darts, neural nanites, etc. That would be something! 2 tons of ceramite / plasteel being wielded by someone mad with rage or fear, who thinks his friends are "really" Chaos daemons or Tyranids. Ouch.

How about critical hits causing mechanical failures? Power armor repair should be difficult and maybe involve building costly, high-maintenance relationships on Forge Worlds, or even epic endeavors to convince power armor technomats to be placed in permanent tutelege under the offending explorer.  There is also The Wild West problem; if the power armor is a relic or sufficiently ancient, it may be famous. Plenty of skeazy and shifty low-lifes are looking to make a name for themselves by killing the owner of St. Cripsin's Holy Vestments. Most of this scum isn't inclined to fight fair, so they are gonna try to kill him while he's on the toilet.

Also, Psykers. A good Psyker can make mincemeat out of the brains of almost anyone in power armor.

 i'll say it again: power armour doesn't make someone a threat nearly as much as what weapons are carried. a technical with a heavy machinegun is a far greater threat than the armored car that picks up money from banks and casinos

In Rogue Trader violence should be one of the many tools used by the PC's.  Now if one of the PC's has chosen to be that tool (i.e a power armoured arch militant death dealer) then that is great and when the time for violence comes then they should be perfectly entitled to roll through most general cannon fodder.

However as a GM you need to emphasise that violence is just a tool.  A Rogue Traders power is based o ntheir wealth and political influence.  Simply put if a player insists that he is always wearing power armour then start hitting the whole group with Aquisition modifiers.  If they question this then simply say that it is diffficult to finalise a trade deal when x character keeps attending meetings dressed like a walking armoury. 

Like wise the stink, noise and reputation of power armour means a basic -30 Fel when dealing with any social situations that require any kind of tact.

Finally if he is literally always wearing the power armour then slap a massive PF cost on it.  After all he is using a rare and valuable resource (power armour power packs) on a constant basis.  That costs money……

As for combat have found that simply allowing NPC's to do Righteous Fury is the best way of evenign up combats.  At the very least 1 in 10 hits is going to hurt.