1. Making them disruptive or highly unhelpful for the party.
2. Being slow while controlling them in play - being quick is a virtue for everyone, but if you add a PL or AC, you may be roughly doubling the decisions and dice-rolling necessary in any given combat round.
3. Allow the player to largely direct their PL's course of action. They built it, they really should get to play with it. Ideally, the referee should never have to step in.
4. Involve the PL in the plot etc. to a level which the players as a group are happy to roleplay.
5. Not be resentful of or especially hostile towards the PL, and especially not single them out in combat - the player may be just as attached to them as their actual PC.
6. Outshining any party member at what they do.
7. Abusing the "Veteran" ability. Some would argue this shouldn't be available to PLs at all; to me, it certainly seems a bit much if used to boost the TL of the PL above 1 before level 4, but a grade or two might be justifiable after that point.
8. Above all else, don't be a [abbreviated Richard].
These I pretty much have no problem with. The second point has little to do with player controlled NPCs and more to do with the player themselves: some folks just don't get into combat the way others do. But the chances are generally good that if someone is making the investment of their character options to grab a PCNPC they're not going to be clueless how to use them.
A caveat on #5 is that by their very nature some PCNPCs are inherently going to get themselves made priority targets -- spell casters, certain species, damage potential, etc. A good GC will point this out to the player as part of the NPC approval process.
I'm generally with you on #7 -- keeping PL threat level down is one of the stronger balance mechanisms for PCNPCs. If you're going to let them use it, hit them up for 5 XP.
Pretty much everything else I wouldn't poke at with a stick I'm afraid. They're incredibly subjective.
Taking any ability which grants a feat to the party (e.g. Teacher, Cadre, Trailblazer). Taking a feat to get a feat (and have the rest of the party get it too) is not right.
As a GC, I'm actually more inclined to look favourably upon a party-buffing NPC than one that just solely benefits the player, especially if it fits the NPC's concept. It shows they're at least trying to be a team player.
Taking more than one base class core ability, more than one expert class core ability, or a class ability at a much earlier stage than a party member would have it (e.g. a gamebreaker ability).
- Taking a spell of a much higher level than a caster in the party would have access to.
Sooooo many games never make it as far as level 14, but again it all comes down to what the GC feels
is appropriate (and that's my biggest problem with all this, it feels like a player trying to browbeat everyone else into his preferred play style). If someone's pet gorgeous cleric (only the best, lay on hands, lifeline, mass cure wounds) means that everyone else is free to get on not being relegated to the party medic/buffer, then more strength to 'em.
Hell, having someone around with Lifeline for a low level game where it's still far too easy for PCs to get ganked by a couple of good rolls is an absolutely brilliant
idea. Of course, this lucky charm could easily attract the wrong sort of attention...
- In particular, avoid very high damage-output builds (e.g. archery abuse, natural attack abuse)
Why? What if we're all social and stealth oriented characters who need an NPC howitzer to guard our behinds? Or noobs needing a babysitter? Or dragon riders? Having an NPC capable of inflicting that sort of output doesn't mean they're going to be there all the time to do it.
Taking too much free stuff (a rough upper limit of "a study and a language plus one beneficial 0 XP ability for every 15 XP the character is worth" might make sense).
Someone like C-3PO is a classic example of an NPC with "All the languages!" in effect. Moderation is generally a good idea yes, but like everything else this is, again, very context dependant and entirely up to the GC to decide if it's appropriate.
- Taking the "story-critical" ability. The campaign should not revolve around an oddly unkillable NPC tagging along with the party - that way madness lies.
IF the GC don't want it, you can't have it. Personally I have problem with it, but I think it is probably underpriced for what it does (should be 0 xp for 1/adventure, 5 xp for its current functionality)