Well.. to be fair - the only two (three?) complaints I saw of yours that could be considered "poorly implemented" is you don't like the way SMS works (some people do, this is personal preference), You don't find an easy way to navigate contacts (which, with keyboard I think you can just start typing, without I think there is shortcuts... but you obviously don't like these methods which again, is personal preference and you have that right, and you even say "IMO"), and a lagging screen when rotating.

Everything else you mentioned are features that were not added, and are covered under the fact that "some phone features" does not mean all phone features were added. For example:

MMS, USSD, Favorites, APN's, separate ring-tones per contact.. etc.

Those features were simply not added.. and thus cannot be said to be "poorly" implemented. That is why people argue that to look at the N900 as a "phone" is prone to failure because it does not come with what has become the "norm" for "smart phones".

The N900 does many things the iPhone and Android do not do... and the iPhone and Android do things the N900 does not. The latter are mostly phone features, and apps. The former are mostly more PC related functionality.

Thus, to look at this as a Linux device that makes phone calls will get you more accurate than looking at this as a Phone that runs Linux. Both are technically accurate, but the latter implies certain things to most people that the former would not - and creates a lot of the complaints we see around here.