The wonderful gnome mage blog, Gnomeaggedon asked, When is your DPS too low? Now while I won't go into exact numbers, he brings up some questions in the article I'll take a stab at.
How/against whom do you measure it?
As a mage I measure my DPS against everyone in the raid. Traditionally, us mages have been at, or near the top of DPS meters. There's no reason why we shouldn't be above any other class. Recently, the developers have stated that the "pure DPS" classes, rogues, mages, hunters, and warlocks should all be about equal atop the damage meters. Unfortunately, that hasn't been the case for quite a long time. I keep up on all the recent theorycrafting and I have the optimal spec with the right gems and the right amount of hit rating and all the best enchants and yet, there's little I can do against a rogue these days.
Similarly, feral druids can consistently out DPS me if they know what they're doing which is quite confusing since these "hybrid DPS" classes should not be able to out DPS any pure DPS class. At the present time, I'm pretty confused as to the state of mages in the game as there are many other classes with the ability to out DPS mages, though you always have to be careful as you go from fight to fight.
What fights are acceptable measuring posts?
I think what you're looking for here is the single target, limited movement, pure DPS races. Or, in other words, Patchwerk. There's no movement, there's no threat ceiling to worry about if you have competent tanks and you can go all out to the best of your ability. On our final few Naxx25 runs, I was always in the top 3 or 5 depending on RNG of crits, but still, it's quite disconcerting to see feral druids top you and know there's nothing you can do to top them because your gear just isn't dropping like you had hoped (again, it comes back to RNG).
Trash I throw out. Each pull is so vastly different that you can't ever measure DPS from trash. In Naxxramas, it was pure AOE, in Ulduar, you have sheeps, you have spell stealing, you have to bunch up and get interrupted, so trash is never a way to measure DPS in my opinion. Similarly, because there's trash in a full run, you can't measure it there either.
So what do you use as a measuring post? I'm finding in Ulduar, there is no answer. In the end, it comes down to raid utility. Are you properly decursing if necessary? Are you killing the trees on Freya or breaking the ice blocks on Thorim? Are you targeting the right mob and giving it your full DPS or are you AOE'ing when you shouldn't be or hitting Freya when she's healing each tick?
What can you do?
I pretty much agree with my fellow gnome here. Stay alive, burn every cooldown you have whenever you can (and if you're using a mana intensive spec, know when to use evocation (at the end of a heroism or icy veins)), and play around with dual specs.
If you're not alive, you can't do any damage. If you can't stay alive, you're leaving decursing to either another mage or a druid who should be healing. If you're dead, you might hope for a druid to pick you up, but it's not likely. If you're a Frostfire mage, you have multiple cooldowns to use. You've got your mana gems, icy veins, combustion (if talented), and mirror images (if they can stay alive, it's worth using the global cooldown). So you just need to essentially use them every time they're up, except under certain conditions. For example, I find that my mana gem and Icy Veins cooldowns are quite similar and it's optimal to use these two together. Why get hasted frostfire bolts if you can get hasted frostfire bolts with extra spellpower? If you just have to wait 10 seconds to line these two up, I think it's worth it (though I have no theorycraft to back that up). Similarly, if you have Molten Fury and your target is at 37% when Icy Veins is available, wait a couple seconds until you're under 35% and you can do 12% more hasted damage.
What do you do in the end?
When all is said and done, if your group takes the boss down, you've been successful. In the end, while I look at the DPS meters and sometimes some idiot (usually the person leading the meters) will link them for the raid to see, they're worthless. Congrats, you were able to stand on the boss and DPS longer than me. On XT-002, while you were there getting in your optimal rotation, I was AOE'ing the trash from the scrap piles down. Without me, you wouldn't have killed the boss. While you were sitting on Freya during her final phase doing insane DPS, I was switching to the trees to prevent her from healing and hitting the enrage timer. While you were sitting in one place on Kologarn DPSing the hand and torso at the same time, I was hitting only the hand, AOE'ing the rubble, AND running away from eye beams. In the end, meters mean nothing. As long as you contributed damage and the group took down the boss, you are a success.
The original post was "When is your DPS too low?" Well, while there definitely is a low end, as long as you continually work on improving your DPS, your raid shouldn't have an issue with you. If you're running heroics on off raid nights to get gear that will help your DPS and doing dailies to buy materials for the enchants needed to maximize your DPS and contributing to boss kills on a consistent basis, what do they have to complain about? Maybe your rotation could be tweaked, that's why it's always good to have someone else to bounce ideas off of. Maybe your gemming could be improved - ask your fellow raiders to take a look. While I always try and do the best DPS and/or damage on each and every fight, it's a sad fact these days that mages are not going to be at the top of the meters, but when you look at all the utility we bring to a raid, that's fine with me (well maybe it's not...but...).
So maybe I've been rambling about DPS, maybe my points aren't the most well laid out here, but essentially I just wish people would stop obsessing about DPS and stop trying to top the meters at the cost of others in the raid. If all you care about is DPS on a single target when you could have been better utilized on adds or could have been decursing or sapping or sleeping, then you're not giving it your all.