UT2003 was not that different from UT, really. The major difference was that UT2003 added the double jump feature, apart from that: a slightly faster game feel and some adjustments to the weapons.
In my opinion, UT2003 was better than UT. And the reason why people don't like it I believe has to do with the fact that it is slightly more difficult - harder to hit, and less weapon impact.
Also: in my opinion the changes in UT2003 have little to do with Quake gameplay (particularly as concerns the physics, which remain superior in Q3) The reference to Quake only indicate a lack of references: if it is not like UT, well it has to be more like Quake... what else...?
2 hours to go for me...