The most obvious & complicated feature of all living organisms is:
(1) The ability to sense their surroundings or environment and respond to these environmental stimuli.
(2) Reproduction - sexual or asexual - for production of progeny of own kind.
(3) The ability to growth in size due to cell division.
(4) Presence of complex organs systems such as digestive and nervous system.
(1) All living organisms can sense and respond to environmental cues and these responses are critical in maintaining homeostasis. The responses are directional and meaningful.