“Dye each piece of your armor a different color.”
Contra the OP’s post, we do know a number of achievements, or at least we’ve heard Blizzard peoples describe things they thought would make good achievements. There are also a lot of quite likely achievements that can be projected based on SC2’s. See the blue quotes and more in the DiabloWiki.net achievements article.
Elsewhere, Bashiok replied to a couple of questions about Inferno perks. The first question asked if the 18 tiers of armor gear sets included the Inferno-only stuff, or if there were more. e.g. are the long-established 18 gear sets distributed throughout norm-nm-hell, and then we’ll see 2 or 3 more just in Inferno? Or did the devs just redistribute the 18 sets, so now we get say, 1-16 up through Hell, with 17 and 18 mostly/exclusive in Inferno. The later, says Bashiok.
Another player asked if all Inferno monsters are lvl 61, or if some are higher. (I’d assumed they would progress a bit, or at least that some special bosses would be higher than 61.) Bashiok’s reply was brief:
It seems weird that all Inferno monsters would be exactly 61; after all, there’s no way the shambling Act One zombies will be as hard to kill or as dangerous as some caster mage thing you’ll meet in late Act Four. So why should they be the same Mlvl, or offer the same sort of item drop potential?
Then again, we don’t know what Mlvl determines in D3, and won’t for some time yet. We know how pretty much everything about how D2 works, (and lots of stuff works oddly due to minor bugs) but that’s thanks to years of labor by lots of code digging genius Diablo community members. It will be a while before we obtain similar insight into the intentional and accidental game functions of Diablo III, an info-delving a process that’ll be no-doubt slowed by Blizzard’s blanket ban on fan-made mods and D3’s online-only DRM.