A quick photo to illustrate why it's worth keeping tomato seedlings inside (or in a greenhouse) for that little bit longer, rather than just dumping them outside once they've grown their first couple of leaves and been transplanted. These are the same type of tomato and were sown at the same time:
Transplanted and put straight outside
Transplanted and given another week on the windowsill
Not only does the indoor one have a healthier colour, it also has an extra pair of leaves. I'm hoping that the outdoor one will pick up in time but it'll certainly take longer to reach fruiting stage.
I am considering a further experiment by picking one plant to put straight out from the windowsill without hardening off, and comparing that a week or so later with its hardened-off siblings.