For six years I lived among the Thai, and now since 3 years I am living among the Isaan. Isaan is a region of Thailand, the northeast, but it is much more than that. The Isaan have an identity that can clearly be distinguised from the Thai.
Of course, the first question is: Should you write about Thai and Isaan as two different ethnic groups? Or are the Isaan a subgroup of the Thai? The Isaan themselves are clearly talking about it as two groups. They are talking about the Thai (so not the Central-Thai) language and the Isaan language. And you can hear things like: “My mother will not allow me to marry a Thai.” So an Isaan is a Thailander, but not a Thai.

Yet there is a growing sense of national unity. Among the older Isaan generation it is still common to talk about the ‘Lao on that side’ (i.e. Laotians) and the ‘Lao on this side’ (i.e. Isaan). They feel more connection to the Laotians than to the Central Thai. But for the younger generation this is not true anymore.
I have to admit that, even though I am a cultural anthropologist, I did not extensively study the differences between Thais and Isaan. Here I list the most significant ones from personal observation.

Language
The Isaan have their own language, and that is important to them. The difference between Thai and Isaan is about as big as the difference between Spanish and Portuguese. It is mutually intelligible with some effort and practice, but the differences are significant. In any official setting, Thai is used exclusively. In any other setting, it is always Isaan. Isaan is not a written language, but it remains the heart language of the people.

Food
Part of their identity, the Isaan derive from their food. They eat sticky rice every meal, and papaya salad most meals. Part of their pride is to grow their own rice. Most people who have a job, still have a plot of land to grow rice. ‘Living off the land’ is also important to the Isaan. A real Isaan will catch fish, lizards, frogs, and beetles. Most meals will include leaves (either herbs or tree leaves) which give the meal its distinctive Isaan flavour.

Living
Compared to the rest of Thailand, the village in Isaan is more important. That’s already visible from Google Earth: Isaan villages tend to be very compact, while Central Thai villages are far more spread out. I suspect that is because Isaan is much drier than Central-Thailand. Villagers need to talk with each other about water distribution. They are much more dependent on each other than in Central-Thailand.
For that reason, Isaan villages feel friendlier than Thai villages. The flipside might be that the social pressure not to become Christian might be higher (though I cannot compare this from personal experience with Central-Thailand).

Honour
The Thai have a highly developed sense of honour. Therefore they are easily offended. Westerners almost always notice this way too late. When you have offended Thai, they will not talk it through, but will withdraw from the relationship. The same is true in Isaan. However, Isaan are not offended quite as easily as the Thai. In everyday life, this to me is the most significant difference between Thai and Isaan, and makes it a bit easier to relate to Isaan than to Thai.

Minority
The Isaan are more and more integrated into Thai society. When Isaan find work in Bangkok, their children, if born there, will not consider themselves Isaan anymore. So the Isaan identity is somewhere in between an ethnic and a regional one.
Yet the Isaan are still highly aware that they are not the historical elite of the country. Some have an inferiority complex, and it still happens that Isaan in Bangkok will not own up to their background when with Central Thai.