1. dark on face below a line drawn through the eye [yellow line here]
2. concentrated white 'hot-spot' low on back of face
3. peaked crown above the eye
4. short, thin, straight or slightly upturned bill
5. much dusky on neck, including on front of neck
6. rear of bird often carried high [but not always, as in the front bird]

• crisply white cheeks, without any apparent dark below the line of the eye
• a flat-looking crown
• much white to front of neck, and
• doesn't carry its rear end high

This Horned Grebe has a red eye while the Eared Grebe on Crespi has a yellow eye. This is age related (more below)

HOGR photo © D. Roberson 15 Nov 2004 Monterey harbor

My experience with misidentified Eared Grebes is with just these two fall birds in Monterey. Both were yellow-eyed birds. Thus I speculate that:

• This i.d. problem is most prevalent during fall migration and is a problem in first-year Eared Grebes that look 'white-cheeked' at a distance. Maybe this appearance is lost by winter when they are fully in first-basic plumage? 
• What percentage of first-fall Eared Grebes look like this, I don't know. Our attention was drawn to these two individuals because they were on fresh water where Horned Grebes are rare. It may not be that unusual?

• Could some claims of inland Horned Grebes be mistakenly identified first-fall Eared Grebes? Could this especially be true of birds scoped from some distance?
• On the other hand, Horned Grebes are not that rare inland. I dug out my own notes and found that I saw "4-6 Horned Grebes" among "lots" of Eared Grebes on 13 Nov 1999 at Los Banos Creek Reservoir, Merced County. I have no other details but I was with Steve Glover and Rita Carratello; surely at least they got some of these right. So there is a movement, at least in November. Are claims in September or October more suspect?
• Large lakes inland have plenty of records of wintering Horned Grebes. I find in my own notes that I saw "at least 15" among over 2000 Eared Grebes feeding on threadfin shad on Clear Lake, Lake County, on 28 Feb 1999. Surely the vast majority of inland records must be correct.
• But assuming that there could be some error rate in claims of Horned Grebes inland, and maybe particularly in Sep-Oct, what error rate is acceptable? Should all inland counties (at least in northern California) have at least one Horned Grebe record adequately documented by written details (that include bill color), or photos (that show bill shape or color), or specimens? Or does it matter? 
• These same questions apply to early dates of fall migrants from coastal locales. Should we be more skeptical of 'Horned Grebe' arrival dates in Sep or early Oct?

So I'd say many if not all first-fall Eared Grebes have this confusing plumage before they complete their first prebasic molt, which according to the Birds of North America account by Cullen et al. (1999) "continues into October or later," and even birds at close range can be a problem.

That we thought the bird had a straight bill with a white tip, suggests even these features may not be foolproof: If fools like us can get it wrong, others can too. Head shape, and especially the slope of the rump, could vary with a bird's attitude, but I don't know what accounts for this bird's apparently straight and white-tipped bill. The bird was actively diving, so perhaps the bill looked as it did because it was wet. Or maybe the difference between slightly upturned and dark-tipped and slightly down-turned and white-tipped is just not obvious in many field situations.

We should always first consider Eared Grebe when we see a small Podiceps on fresh water, but it does seem especially prudent to do this in September- October. And based on the Madera experience, I think body bulk and  plumage contrast may be as important in distinguishing the two grebes as the features you suggested.