So, you're standing there and one of the members is rigged in ungemmed and unenchanted PvP gear, in a spec that would make your grandmother weep in shame. And swims around in The Green Pool of Doom. While pulling the next group of mobs.
You're now in the position to give advice.
Before doing so, decide what kind of result you want.
1) You want the offending player to vanish.
Your advice should be formulated in the terms of: You suck!
2) You don't really care if the offending player vanishes, but if staying performance had better improve.
Your advice should be formulated in terms of: Your ability to perform sucks!
3) You really do want the offending player to stay, but performance had better improve.
Your advice should be formulated in terms of: If I played your role I'd do the following to have less sucky performance.
4) You really do want the offending player to stay.
You shouldn't advice. Apparently performance isn't an issue.
I doubt there's much to say about points 1, 2 and 4. Possibly that you can get your point through as far as 1) and 2) goes without references to elderberries and other non-humanoid attributes concerning heritage.
But if you truly want the player to improve?
If you're standing there in combat, regearing and respeccing really isn't an option. You'll have to satisfy by getting the player out of The Green Pool of Doom. That is, strangely as it might sound, not best done by telling the player to move the hell out of said pool. At least not the first time. The fifth time you just yell: Get the **** out of the pool!
Point out the non-pool areas. You want the player to focus on the right way of doing it, not on avoiding the wrong way to do it. Even if the pool pops up in your own face from time to time wherever you happen to tank the target, there are still non-pool areas. Often behind the target.
The same strategy goes for hepling with specs and gear, with one huge difference. If you don't really know what is the best setup for the player, then you should preferably introduce that player to someone who does. Don't guess and give wrong advice.
Now, if you do indeed know the best setup, then you could say, to take a prot pally example: You should place two talent points into Seals of the Pure. It provides both higher overall dps and better reliability than Eternal Glory. You want that reliability during the first seconds of the fight when you still need to establish aggro. Later on threat becomes a non-issue, so reliable extra threat is prefered over random spikes of threat.
The same goes for gear: My primary job as a tank is to survive and keep aggro. The reason I prefer the tanking chest enchanted with 75 extra stamina is that it helps me to survive. While your dps chest with +20 on all stats give you better initial threat than mine does, the extra threat gained is unlikely to help you enough during the first seconds of the fight to compensate for the extra damage you take for the rest of the fight. Besides, occasionally those 55 more stamina I get allows me to survive an attack which would otherwise have killed me. When I go down I generate zero threat.
Start with the short time goal you want the player to achieve, argue for it, and only after that point out the deficiencies standing in the way to reach that goal. That's the roundabout way of getting the player to suck less without resorting to: "You suck!" And the elderberries.