In the time you were gone, there were several major changes to combat.
First, in version 1.4 there were several noteworthy changes.
1) Weapon penetration and subsystem damage was changed. Subsystem hit boxes became smaller and weapon penetration was severely reduced, making it harder to damage subsystems.
2) The powerplant subsystem mechanics were changed. Bringing the powerplant of a ship to 0% no longer always resulted in an instant kill. Instead, at 0% powerplant, a ship's power is halved and there is a chance for the ship to explode based on the amount of damage done past the 0% point. This meant that continuing to damage a powerplant past 0% would eventually blow up the ship before hull went to 0%. However, with the changes to weapon penetration and subsystem damage, along with an increase to powerplant hit points, the amount of damage and time needed to bring a powerplant to 0% was drastically increased and for some ship types and firing angles became very close to the amount of time and damage needed to reach 0% hull.
3) NPC AI flying skills were improved, resulting in NPC ships being able to spend a lot more time keeping their front facing the player's ship. This meant more head to head battles of attrition and because of the changes to weapon penetration and subsystem hit box size, also meant that less damage would make it through to critical subsystems.
4) A bug in the NPC AI was introduced that would cause a NPC ship to occasionally stop firing weapons, stop maneuvering, and basically just stop and roll continuously for awhile before recovering. The occurrence of this bug seemed to be related to damage done, as doing a lot of damage quickly (alpha strike) or doing a lot of hull/subsystem damage would result in this bug. This bug would effectively make the NPC ships sitting ducks and caused many to complain that the difficulty had decreased overall, and not just because of the bug. This would lead directly to the changes in version 1.6 / version 2.1 in items #1 and #2, which some, myself included, would say that item #2 took the difficulty a step to far.
Noteworthy changes between v1.4 and v1.6/2.1:
1) Shield cell bank mechanics were changed so that using a SCB would temporarily overheat your ship for a few seconds, causing module damage and at extreme levels hull damage unless a heat sink launcher was used. This was implemented due to complaints for PVP players who were tired of battles being ending in one player fleeing when they ran out of shield cell bank charges, without examining the underlying problem of PVP players being mainly frustrated with their targets successfully fleeing. Additionally, a timer was added to module boot up so that modules would not be available for use immediately after being turned back on, so as to prevent players being able to use more shield cell banks than their powerplant had capacity for by turning on additional SCB's that had ammo and turning off out of ammo SCB's without a great deal of forward thinking.
The noteworthy changes in version 1.6/2.1 were:
1) The stop and roll bug (item 4 from v1.4) was fixed. NPC ships would no longer suffer breaks in their flight pathfinding
2) The NPC AI flight and combat skills were increased again. Now, not only will an NPC ship make effective use of shield cell banks, power management, keeping their front oriented towards a player ship, they now constantly strive to stay behind a player ship, counter the fly backwards and pitch in loops maneuver used by large and slow ships, and minimize the amount of time that a player ship's weapons will fire on them (including turreted weapons!). This resulted in combat in big ships becoming more about jousting with a hostile ship (boosting forward as the hostile moves to get behind you and then coming about to fire on the hostile before having to boost again as the hostile flies past to get behind you, repeating this until one ship is destroyed or forced to flee).
3) Although NPC's are now using shield cell banks, they are not suffering from the changed heat mechanics in that they often appear to not take module damage and they certainly do not restrict their weapon usage as a player would to limit the amount of heat damage.
The major difficulty increase came about as a result of the developers listening to demands to improve the AI following the stop and roll bug, by both fixing the bug and increasing the AI skill at the same time, without checking to gauge player reaction to the first AI skill increase once the bug was fixed. The ideal progression would have been AI skill increase with stop and roll bug introduced v1.4 followed by fix the bug v2.1 and leave it for awhile and then if feedback was still too negative then increase it in the next version v2.2 Instead we got increase AI skill and introduce bug v1.4, fix the bug and increase AI skill v2.1 and then check feedback. And with traditionally forum fashion, the best players are demanding still more difficulty increases while shouting get good or your doing it wrong at anyone who disagrees that the difficulty needs to be increased, or to them, heaven forbid decrease the difficulty back to v1.4 except with bug fixed to allow for a more diverse player skill level crowd to have fun.
