With Apple, you only have one option: the iPhone. This is why everyone is so critical is because it's the only Apple phone you can buy. You could go out and buy around 100 different Samsung phones, compared to the potential of 6ish with the iPhone.
Apple releases 1 phone a year, everything is riding on that phone. Samsung releases heaps of phones a year, so if the S4 doesn't go well, the Galaxy Note 3 can pick up on sales. There's more option with Samsung phones, which is why having 4 million Galaxy S4's sold in 4 days is great, because Samsung still have cheaper phones on the market which people will buy.
With Apple, there's no option. You can only get an iPhone. With Samsung you could get a Note, Galaxy phone etc. That is why people are tough on Apple, because unlike Samsung, they are only releasing one phone a year.