Now, I know runewords wouldn't be implemented over night, and they shouldn't be, but consider what community-led issues you could simultaneously address with them:
- Gems don't allow enough choice when socketed into items
I hear this point brought up now and then, and it's definitely true. Gems only offer players a chance to add on to main stats, but there are dozens of other stats that players like to focus on. Runes that add to runspeed, pickup radius, resource regeneration, etc. would add a great twist to sockets in general and would go a loooong way towards addressing player gripes about itemization.
-I like running bosses, said no one, ever
If runes only had a chance to drop from bosses (and rare spawns, rare spawns never get any love), you would immediately see players change what areas they target. The game has gotten a bit too repetitive, and by incentivizing boss kills (with 5nv), you'll open up so much more content to farming, which is a good thing. I think you would also have to change it so that bosses spawned even if you were on a later quest, as some have suggested.
Also, Act IV is a bit weak right now. You can help that by giving Diablo the highest chance to drop a rune, adding more elites to the act, and making the key warden drop a random key. Also, Whimsysire needs a boss, like the cow king before it.
-White items are pointless
Right now, white/grey items are simply a relic from D2. If diablo 3 was the first of its kind, those white items probably wouldn't be there. But, back in D2, you had to have a sharp eye out for these, because they had the potential to become great runewords. This game sorely needs that aspect added back to the item hunt. I think you can make some improvements here and there to help newbies that aren't familliar with the system, for example by perhaps giving these socketed items their own text color.
- People only wear legendaries now, AKA itemization is bad
Patch 1.04 went a long way towards making this a better game, but a lot of people are still underwhelmed when they think about what it has done for itemization. With runewords, you can take the sprit of that patch a step further, by introducing new items that offer new abilities, new choices, and perhaps bonuses for skills & builds that may currently be unpopular.
One good example here is shields. Very few people use shields (especially not ranged classes) because inferno is no longer as dangerous as it was at launch, and because the alternatives allow the player to do much more damage. New runeword shields can address this by perhaps featuring stats that weren't on shields before, like critical hit damage, or by giving interesting new abilities, like reflect damage.
Like I said before, I feel like the game is in a good place right now. I could criticize account bound items or criticize events for being worth nothing more than an NV stack (when they sounded awesome at launch), but the truth is...diablo 3 is pretty fun right now. Yeah, I said it! The thing is, it's also too repetitive, and there are parts of the content I haven't enjoyed since launch. Runewords would have a massive impact on the item hunt, and would therefore make farming more rewarding and more diverse.