What is it like? First, combat has an extremely lethal feel compared to AD&D as you will see below.
There are no locations or crit charts. Despite that, there is some flavored detail that AD&D lacks.
Traveller does not have increasing level based benefits like hit points. Your "hit points" are your stats (Strength, Dexterity, and Endurance) and if you get damaged in combat, your stats go down and so do your bonus DMs releated to the stat. So as you get wounded, your combat abilities are impacted directly. Better than generic hit point without too much complexity added.
Barring scientific gadgets, enhancements and such, a character at most can take 45 points of damage. The average Joe has 21 ponts. A fist does 1d6 per and a simple pistol does 3d6 or more. Portable energy weapons can do 16d6!
Characters go unconscious if two of the stats are reduced to 0. So characters can potentially die after only 2 pistol shots. Most player have characters with average Joe stats.
Armor subtracts damage points from attacks and does not alter the "to hit" roll. However protection is on the order of one to ten points, mostly, so characters are still not so likely to survive Fusion and Plasma Guns...
No attacks of opportunity. Actions are categorized as major, minor, free, and reactions. Characters only get one attack per round (major action), but can potentially dodge multiple attacks with increasing penalty to the dodging character.
In hand-to-hand characters can use their skill rating to act as a -DM (parrying), but you have to decide how much for attack and how much for parrying.
Shields, depending on the rules used, either adds to parrying (Core Rules) or subtracts from damage like armor (Supplement 4).