The indifference of the Laodicean Christians is alluded to metaphorically in terms of the [water] temperature their deeds betrayed. Significant, perhaps, in directing this criticism at the Laodiceans in particular, was the well known sources of hot (Heiropolis/Pamukkale) and cold (Colossae/Honaz) water a short distance to the north and southeast of the city. Whereas there were good, obvious uses for hot or cold water, tepid water was all but useless, as the Laodiceans could well attest. In calling these Christians lukewarm, Jesus was indicting their uselessness which resulted from their indifference toward him.
How did they get to be in such a careless state? The reason implied in the message was that their satisfaction with earthly wealth had blinded them to their spiritual poverty. Laodicea had experienced a devasting earthquake in 60 C.E. and was knocked virtually flat. The wealth of the city was such that its inhabitants rebuilt it without government assistance, recovered relatively quickly and continued their prosperous ways afterward.
It seems the Laodicean church could have been interpreting that history as a sign that they were blessed by God and just fine in his sight. They were mistaken. They did not remember that we cannot serve God and mammon, for if we serve the latter we'll end up despising the former, and be useless to him. As Jesus saw it, they actually wretched, miserable, poor, blind and naked.
Jesus counsels them to come to him and engage with him (i.e. "buy from him") rather than to be satisfied with the bounty they got by their own devices--a rather ironic command considering he just said they were wretched and poor. The gold, garments and salve they were able to procure pursuing their own worldly agenda was actually not a sign of blessing but merely the wallpaper over the pits and cracks of their profound need. Though tangible, the earthy is no substitute for the spiritual, and when one has that aright, there is no need for wallpaper.
I can't help but see, in this letter, a dire warning to the Western Church, first as it existed in Europe and now as it does in America. That church, in large measure, discovered a marketability in the Gospel and has been producing a wealth-generating, product-oriented salve for the guilty consumer for much of its history. Whether depending on a few rituals (e.g. mass, baptism, confession) or a few works (e.g. church attendance, tithing) or just grace, this church sells the convenience of a guilt suppressing palliative, without the necessity of taking up the cross and walking with Christ.
If salt has lost its saltiness it's good for nothing; if living water is not different from the world around it (for instance, in temperature), even it is useless. If those of the Laodicean ilk, useless to Christ, indifferent and thereby in peril, can but open their eyes and remember that they are nothing, and have nothing without Christ, their recovery begins. The world and its riches offer nothing of true value to any of us that we should find it satisfying. Jesus has all we need, and all that is worthy, and he's willing to give that to us if we'll come to him.