The personalisation of search results throws up a whole heap of questions which can be debated till the cows come home. But for me, the main two which need to be discussed are privacy and user experience (broad I know).
Google announced in February that it would be making a change to its google account sign in process to allow for more personalisation of search results. Basically, as far as I can gather, the change will be to make the opt out of automatic sign in box as difficult to find as possible! Im sure there are certain legislation they need to comply with but this act throws up a whole heap of arguements (which I dont really want to go into). From my investigations into personalisation (though limited) I know that it can spook people. If you dont understand the automatic sign in process and google starts telling you what you have previously searched on and what you may be interested in it can be a little big brother-esq, producing a negative user experience.
Further affects on user experience can be produced if google interprets search data based on irrelevant criteria. This has been one of the pitfalls of local search. Basing results on a users location is great if they are searching for a locally based product. But what about if there product doesnt need to be locally based, financial services for example. You dont need you mortgage lender to be based nearby because you can perform all the necessary actions over the phone or online therefore making the search results less relevant than the normal ones. Similarly with a product such as hotels. a man in manchester searching on hotels is unlikely to actually want a hotel which is in manchester, more likely he wants a hotel in a city further afield for a weekend stay for example.
Im sure that the behemoth that is google has considered all these problems and will roll out something which addresses them all. But until this is the case I have my reservations about it taking off. I am all for it, believe me, but just think there are some rather lage hurdles to overcome for the product.