I think a smartphone needs to have a solid list of basic features right out of the box - a solid stable UI, an enjoyable UX, and good calendar, email, tasks, memos, and SMS/MMS built-in apps.
The iPhone has a good mix of all of these, as well as a strong set of apps, but it's only available in one form factor. Android has multiple form factors but a mediocre UI/UX. A user can hobble together a usable Android smartphone by using all the third party apps to compensate for the missing built-in functionality but the results aren't always as stable as they can be with built in apps.
WP7 has a great UI/UX, great integration of email and, in Mango with workarounds, calendar support. The third party apps are growing and MS seems to really be working hard/spending the money to make it grow in the areas that consumers want. I wish tasks and memos were handled better, as I use these constantly for work. I know there are third party apps for both and there is the synchronized OneNote app, but that requires opening the Office hub and then OneNote. I would like for OneNote to be pinned to the start screen, along with Tasks. (I think that might be the case in Mango, FYI, but I'm not sure)
Anyway, back on topic, apps are important up to a point. If one maker has 100,000 apps and another has 300,000, does it really matter? With numbers that large, most, if not all, of the most popular apps will exist in both environments. As stated earlier, the top 50 apps make up well over half of those used so the real key is to have the RIGHT MIX of apps, not the most. But just as megapixels became shorthand for how good a camera is, the number of apps has become shorthand for how functional a smartphone is. The reality is that I'd just as soon not have 10,000 fart apps available to me but I don't want to deny other the subtle variations and tonal qualities found in the plethora of flatulence simulators in the Android and iPhone app stores.