Thinking about it, some of the more specific/less-obvious 'worsts' that come to light, after the fact... though as I scrape my brain, I'll just drop the one concrete event.
-Botching the deal with Sun. I *still* haven't seen Deathbed Vigil, but done right, this would've solved many credibility issues, whether it was a cloning license or a way to shift 3000UXen.
The 'most ambiguous' awards go to:
-~640x200. From what I gather, chipset support for 'delaced' resolution was a possibility early on in the game, while the chipset was still being finalized, but Commodore nixed it based on CRT (and RAM) prices, and, I'd imagine, their existing inventory. Had the option been demoable- even if not affordable- at launch, or perhaps included for the 2000, would things have been taken more seriously by the DTP market?
-The 900. The A1000, as shipped, was most definitely a 'Personal Computer,' albeit mindblowingly advanced; the 900 was most definitely a 'Workstation.' Could focusing on a multitiered strategy- 900 for corporates, Amiga for the 'midrange,' and legacy inventory/terminals for the low-end have prevented the death spiral later in life? Or would it've just created a mess worse than the menagerie of almost-compatible 8-bits?
-All-In-One casings. A boon to the users who could get a capable machine cheap, but a surprisingly lasting image problem (which, coupled with the default NTSC resolution and WB1.3 color scheme, led a friend to remark that his garage-sale 500 appeared 'more primitive' than a IIgs.) Would Commodore have crumbled without the (-or with a delayed, e.g. OCS at the launch of ECS) 500, or would holding off have forced the community into sync with the normal world's disdain for 'little boxes?'
-CAOS. What if, somehow, some way, the vision was fulfilled? Would it have meant nirvana, pricing out of the market, or performance/memory issues that would've been crippling in 1985?
-Telecom. Online services were outrageously expensive in the day, but from my memory, they could really put lipstick on a pig. Had the Amiga been stronger in this point (through the dealer network- Radio Shack certainly played a role in selling us an unaffordable Compuserve account back in the day with our 1000SX), and/or if, say, some effort had been made to lure Steve Case back to the fold near the endgame, could things have somehow been made to pull through? At least any of the later STB ventures might've been left with an obvious partner.
(I seem to remember reading Compuserve access was part of the demo at the launch... but through the early-'90s era, was there any sustained initiative on the order put forth by clone dealers? What if they'd offered a promo with the CDTV?)