Just to add another viewpoint: MQTT seems to be a very popular choice for implementing new sensors. I have worked with MQTT-based sensors before, but when I started building temperature/humidity sensors, I decided against using MQTT.
My rationale is simple: I need another component (the broker). The broker can go wrong at any time, and for mosquitto (which seems to be the most popular choice) I saw some outages at work. On the other hand, with the readily available webserver components for the ESP8266, it is very easy to create a sensor that can be queried just via HTTP and discovered using MDNS.
HTTP is a component included in OpenHAB, it is well-understood and easy to use. So for me, it seemed like the better choice. But don’t get me wrong: I’m perfectly fine with other projects going different routes. In fact I think it is interesting to see how different ideas compete and evolve. The nice thing about OpenHAB is that it simplifies the integration of very different protocols, so even if you decide to abandon one technology and go for another one, you can keep your rules and datasets.