I’ve seen so many people lately hanging onto all their ICC gear… even the 251 4 set because “I don’t want to give up my set bonus”.
Drop them. What you’d be sacrificing to keep the set bonus is far too much in the way of stats.
Seriously, I started replacing most of my main’s gear at level 83 in Deepholm, replace more in Uldum and pretty much the rest of it, save the Ashen Verdict ring in Twilight Highlands. The couple of alts I’ve taken through the first 15-20 Hyjal quests have replaced 3-4 pieces of their gear IN THE FIRST ZONE.
I know it’s a hard thing to realize, especially if you haven’t been through a gear reset before [and each reset is also slightly different – in Wrath upper end BC epics would take you into the first raid], but refusing to replace gear simply for an old tier set bonus only gimps you and consequently your heroic groups/raids.
A tank keeping their t10 4 piece bonus doesn’t have the health or the mitigation to take the beatings of instances.
A healer keeping their t10 4 set doesn’t have the mana or the mana regen to do their job.
a dps keeping their 4 set doesn’t have the dps to do their job.
I’m not saying to upgrade to whatever piece – but compare stats and upgrade accordingly. Don’t worry too overly much about perfectly balancing stats till 85 [hit/expertise/agility/crit] – just make sure you have enough to do your job – aka don’t run around with zero hit rating. If you keep an old piece, choose to do it and know why you are. For example, for my hubby’s ret offspec, he’s still using the 245 hit trinket simply because most of the offspec gear he’s managed to get his hands on is devoid of hit rating and he hasn’t managed a current hit trinket yet. He knows it needs to be replaced and is just waiting on the right trinket to drop.
Does it feel wrong to replace epics with greens? Yeah, a bit… but it won’t be too long till we’re replacing those greens with blues and epics. Isn’t it better to have a stat jump than shiny purples?
You Yank It, You Tank it: Blog of a Tanking Death Knight Lady 
Most importantly:
Anyone wearing their T10 set bonuses just looks tacky when compared to all the other freaking awesome gear models there are now! Who wants to run around Heroic Throne of Tides when they look like something the Lich King pooped on?! Come on, people, FASHION IS EVERYTHING!
(This comment is somewhat tongue-in-cheek… and somewhat not… it’s a new expansion! Embrace the newness! And that means new underwear, Mr.This-is-what-I-downed-LK-25-in!!!)
Nevermind the simple fact that most of Cata 5mans consist of a lot of AoE damage. As this is a present and serious concern sticking to old gear means a LOT less Stamina to survive that.
If your 85 and not sitting right around 100k health as any class you might need to re-asses what gear your holding on to.
Yes, whenever I have to replace a blue or purple with a green, there is a tiny little piece of me that goes “awww, man.” Seriously, though, a stat boost is a stat boost. As you say, our gear will gain the desirable purple pixels again in due time. In the meantime, I’d rather have gear that makes my duties easier.
If Lich King’s parade of purples has taught us anything it should be that itemization is far more important than whether it is rare or epic, and to a lesser extent even its ilevel. There were i200 blues that outperformed any i200 epics and some i219 gear just because they were so well-itemized. Of course sockets on gear also make it far more valuable than any of its same-ilevel peers without sockets, especially when you can shove in gems from later expansions. There are BC blues you’ll get in Hellfire that you can safely keep for at least 10-15 levels if you throw in cheap wrath or cata gems.(Yes, sockets are that OP).