I have been playing since Nov 03. So not quite from the beginning, however here are some of the significant game changes it that time.
Vehicles. These were introduced into the game, meaning you did not have to run everywhere (hell, I remember running from mos eisley to Anchorhead doing artisan missions for 2k a pop!!!)
Doc Buffs. These have now been changed twice that I know of. At the start, buffs were random, ranging from 500 - 4000 per stat, dependant on packs used. Many complained about this as you could do a health buff of 4000, then an action buff of 800. Was kinda fun when u hit the 4000 tho.
This was changed to the set amount method, where depending on the quality of your packs depended on the buff. Nothing random, just a set amount each time. (and gone were the really high buffs).
Image designer. This got a revamp, allowing ID's to migrate your stats. Before you could change your own stats but it took a very long time to fully complete (days).
Jedi. The path to jedi has been tweaked a few times. At first it was a random proffession (or series of professions) That unlocked your Jedi. (The profession grinding days). Not soon after I started they introduced holocrons to the game which hinted at the proffs you needed to grind next. (three told you a profession the fourth was silent). The silent one you had to figure out for yourself (Luck).
After this Jedi was given a revamped to the Village (Pub 9). No more holo grinding. Personally I think a bad idea as it gives people a very linear path to jedi, that they can see how far they have got. Also with pub 9 perma-death (blue glowies) disappeared. This was that if a jedi died a number of times he was turned into a glowie that could not react with the player base.
Jump to Lightspeed. This introduced space into the game. Pretty obvious what that did.
The Combat Upgrade. By far the biggest update to date, and a source of much anger from veterans of the game. This completely changed the combat system in SWG from a HAM system where different pools could be attacked to only having a Health bar that took attack damage. Also with this publish, many professions have become kinda useless due to their skills not being needed anymore. For instance - Dancer and Musician. These 2 profs do exactly the same thing now, so no point in having both.
These are the main changes I remember and could have a couple of the details wrong (feel free to comment if I have) if anyone else thinks of others feel free to add them.