If you are a new raider who hit level 80 some point after ToC came out, it's possible you have never seen Naxxramas (beyond weeklies, at least). Ulduar is the best raid instance WoW has ever had, but despite the Accessibility Doctrine, very few people have seen Yogg-Saron. This isn't because those instances are too hard, or because those people don't want to see lore. It's simply because the rewards in those two instances are completely obsolete. Heroic 5-mans give tokens that will buy you gear that blows the drops from those raids out of the water, and some 5-mans even drop gear on par with ToC 10 itself. There's no efficient way to advance your character in the old instances, so very few people do them.
Blizzard could fix this right now while giving players an entry-level raid experience to learn from before going into "real" raids.
Have bosses in "obsolete" raids drop a ton of emblems.
Make it so raiding Naxx is a more efficient way to get emblems (currently, Triumph Emblems) than heroics. The earlier bosses should drop fewer emblems than the later bosses, so going deep into the instance would feel rewarding. Maybe have the final boss also drop a Frost Emblem on top of the Triumphs. This will be even easier to balance in Cata with the move from emblems to points.
And it fits in very neatly with Blizzard's current design philosophy because you can only do it once a week. Instead of having to log in every day to do a heroic, I can get similar emblems from spending two hours at a stretch on one night doing Naxx.
Combine that with the upcoming weekly cap on emblems that forces you to choose how you want to earn them, and it's even better.
For this system, an "obsolete" instance is one that drops gear a tier behind the current lowest level emblem. So using Wrath as an example, Naxx would have gone in the Dungeon Finder with inflated emblem rewards in the same patch that released ToC.
The best way to do this would be to put the older instances into the Dungeon Finder. There are obviously major challenges with putting together, say, an ICC 25 group in the DF. But with content as easy as Naxxramas, it's easily justifiable, especially with the new raid lockout system that let's you have more flexibility. Just have it use the same minimum gear requirements as ToC 5 or the ICC 5 instances so you don't end up with an entire group in just greens expecting to be carried. At the same time, I anticipate most of the people queueing for these will be on the low end of the gear scale, perhaps even being able to use a few "obsolete" drops from the raid in a few slots.
Seeing how Blizzard plans to automatically scale point acquisition as new raids are released (meaning that, for instance, when Tier 13 comes out, Tier 12 raids will go from "frost emblems" to "triumph emblems"), it worries me that we'll see the same abandonment of the early tiers that we saw late in Wrath. And that would be a shame. Giving players a reason to visit obsolete raid instances as an efficient way to advance their character would have a myriad of benefits, from breaking up the monotony of running the same raids over and over, to giving new raiders and new alts an appropriate training ground for raiding, to simply preventing good instances from becoming ghost towns.
I hope they think of it too.