Quite a large one, I'd imagine. The Nook and Kindle are a pretty far cry from the iPad in terms of capability. To me it seems like it's a very close parallel to a smart phone vs. a feature phone.
(Note: that's all if you don't root it, those who do can do pretty much anything on it that android can do)
The Nook (the original one, which goes by simply 'Nook') is an e-reader with a 6" eInk screen (a Kindle clone, if you will).
But the actual hardware is half the story (or maybe 30%), these things will live and die with their respective app stores, which is a point Scroble made in his blog. Currently iPad is lightyears away.
As for the reader/tablet discussion: this will blur in the future. Amazon is getting their app store ready as we speak.
At the early stage in the tablet genre we're at, this is largely moot - clearly Apple have a huge lead, both in sales and in consumer mindshare, and everyone else is playing catchup.
On the other hand, things can change.
I'm sure Sony and Nintendo would be quite happy to define the 'handheld gaming device' genre as one in which devices have buttons and analogue sticks, but saying they've got 90%+ of the 'buttons and analogue sticks' genre isn't much comfort if everyone is using their smartphones and tablets for gaming.
Similarly, Newspapers and printed media will be happy to define 'newspaper' to be something printed on real paper and then claim X marketshare, that doesn't mean that they're not losing out to online news sites.
If in a years time, Apple are saying 'but we have 90% of the REAL tablet market!' then the shareholders should be worried.
At the end of the day, it's still a case of No True Scotsman[1]
Or to put it another way, there are no other products like iPad.
FWIW, the Optimus line of phones from LG is doing the same in the featurephone/smartphone market (priced like a feature phone, but with all the power and features of a 1 year old top of the line smartphone).
They're very close on raw specs, e.g. both have Cortex A8 chips, the iPad running 200Mhz faster, and some of the specs are actually better on the Nook e.g. screen quality or memory (512 vs 256).
It's more like iPhone 3GS versus Nexus One than smartphone versus featurephone.
A Toyota and a Lexus might have the same engine and HP but that doesn't mean they're necessarily competing for share of the same market. Yes, much of that is marketing - but that's the point of marketing.