This is very much the case - in many cases, bad luck with weapon drops can seriously dent the output of an otherwise great melee or hunter DPS. Our enhancement shaman has legendarily poor luck with drops and still doesn't have two ilevel 359 weapons or higher. He dies a little inside each time we kill Baleroc and no Gatecrasher drops. However, weapons can be offset by skill - as evidenced by the fact that despite his substandard weapons (for the tier) this same shaman tends to average somewhere in the top 3 on DPS for the raid over the course of a lockout.
Likewise, certain fight aspects or roles can seriously gimp the output of a given spec. Use caution when looking at Rhyolith for DPS comparison, for example - the DPS for the people on his legs is going to look really poor in comparison to the people who are on add duty.
Basically, be smart about it. There are always a few decent benchmark fights in a tier - not a Patchwork necessarily but at least a fight where most of the DPS are operating under similar conditions. Look primarily to those for your comparisons, if you can.

On our first Lich King kill, the main healer forgot to change out of her dps armor. On our first Staghelm kill I forgot to change out of my hybrid dps/tank gear from Alysrazor. On our first Cho'gall kill, our offtank healer turned out afterwards to be wearing his chef's hat. I actually asked him to put on the chef's hat again when we were having trouble on Rhyolith.

Gear matters far less than player familiarity with content. It matters less than luck. It matters far less than skill. It's nice, but, you know, people cleared normal mode tier 11 in blues and greens.

Yeah. There are always exceptions.

Another thing to take into consideration is to confirm that a bad DPS really is bad DPS. This goes back to Tyler's comment about using meters incorrectly. A DPS who sustains 20k DPS, but ends up dying after a couple minutes because they pulled threat or stood in the fire is worth less than a DPS who only pulls 15k, but is able to do a fairly constant 15k for the entirety of a 5 minute boss fight.

@ DeathPaladin

As the saying goes, "a dead rogue does zero dps." Someone blowing everything and then getting themselves killed because of it (due to either tunnel vision or threat) needs to be reminded of this!

"bad luck with weapon drops can seriously dent the output of an otherwise great melee or hunter DPS"

You say this as if weapon quality isn't absurdly important for casters, too, which it is. The spell power on caster weapons has pretty much the exact same impact on caster DPS as the weapon damage on a physical DPS.

@Boobah: there is a difference
An enhance shaman has a VERY strict weapon list. For some reason Blizzard has seen fit to ignore them again. The craftable weapons from molten front have 0 for them. There is a sword there tho. Had it been an axe, 2 classes could have used it. AND they made a dagger.

The fists from BWD suck. There is only 1 boss dropping one in FL and the lace of fist weapons or maces or even axes is pathetic.

Any spellcaster can pick up something okay at least NEAR the iLevel crafted. But most melee need that extra damage and weapon speeds to get the most out of their weapons. Casters don't have the extra stats to worry about.

@DeathPaladin While I'd consider someone who dies to mechanics or threat early on to still be bad in general, you are correct in saying that you should "confirm that a bad DPS really is bad DPS. This goes back to Tyler's comment about using meters incorrectly."

I actually had to deal with this very situation a few weeks ago. Our raid leader had changed my role in various fights so that I was helping the group but at a significant loss of DPS to me. I'm a shadow priest, and for weeks I'd been at the top or in the top 5 on their meters. Then on Rhyolith I was asked to dps the boss instead of adds. Of course my dps tanked for obvious reasons. I wasn't being sent up for Alysrazor, which of course put me further down from those who were. I was soaking on Beth Heroic, which gave me little time to dps at all (and the raid leader even stated to me that he didn't expect me to). On Shannox I was asked to do Smite dps to help fill in the gaps for the healers when we were short. That was when a couple of the dps decided it was time to call me out on my low dps-- in raid chat.

So I ignored the RL's assignments for me, followed the strats to give me the highest dps, blew the third spot on the meters and beat them both. Of course, the boss didn't go down, but meters was what was important to them, after all. (Fortunately for me, the RL knew what was up and was well aware that I was about to wipe the raid to prove a point.)

So yeah... Consider the bigger picture when asking whether or not someone is doing acceptable dps.