For simplicity, I'll refer to Revised as 2.5.
2.0E: Has full round defenses
2.5E: no full round defenses (excepting shields)
2.0E: Reactions replace range TN only if better
2.5E: Reactions replace range based TN, even if worse.
2.0E: Scaling is table based - 3 tables used.
2.5E: scaling is based upon a simple bonus to number of dice by scale difference.
2.0E: Wound levels: Stunned, Wounded, Incapacitated, Mortally Wounded, Dead
2.5E:Wound levels: Stunned, Wounded, Wounded Twice, Incapacitated, Mortally Wounded, Dead
2.0E: Combined actions increase a dice pool on a semi-log scale from a table
2.5E: Combined actions increase a dice pool by 1d per 3 characters involved.
These changes are very important ones in combat.
The change to eliminate full round defenses means you can't be "evasive" when moving. Well, you can, but only narratively; mechanically, you cannot.
The use higher of 2E defenses (Dodge, Parry) means that dodges at long range are logical. Under 2.5E, since they're reaction only anyway, you only bother once hit. This impacts the narrative with most players, and in a manner that precludes the kind of caution seen in some scenes of both ESB and RotJ.
The scaling change is HUGE. It both radically affects the odds and has a much different feel in play. A 6d starfighter weapon versus a hull 4d capital ship is going to damage capital ships under 2.0E; the average roll will be 15 vs 14, and the flat spots 15-18 vs 12-16; this shows that it will usually damage the capital ship, as it will usually get 3 points on the die. Under 2.5E, the average rolls are 21 vs 35; the flat spots will be 18-24 vs 30-40; in 2.5E, damage to capital ships with anything less than massed fire is pretty hard to achieve. Moreover, combined with the wild die, capping does require more 6's for the same bonus, making the effect of wild dice less in 2.0E than in 2.5E.
The combined fire rules are table driven in 2.0, don't have a cap on the number commanded, and use a logarithmic scale (Using the 2/4/6/10/15/25/40/60 sequence, one die per step). This allows for mass battles to be played out, and covers coordination of fire of even entire batteries of weapons. The 2.5E caps the commanded at a number equal to the dice in Command; never having seen a skill in play exceed 12D, this limits severely the effects of combined actions. This limits the ability to blow huge ships with small vehicles severely; the linear bonus also means the maximum bonus is typically only 2D. It is no longer suitable for mass battle resolutions. At the allowed levels, it pretty much only drops the bonus by 1D.
The extra wound level is almost ignorable; it minorly increases survivability.