It certainly doesn't hurt that they've gone and traveled to Paris for the location filming this time around. That slow pan across the flowering trees to show the Eiffel Tower is a fabulous shot, and it's clear that the Doctor and Romana are having a great time. The scenery is pretty, and I at least don't have a problem with the production team indulging themselves with shots of the Time Lords exploring Paris, given how great it all looks. And the script sparkles, with tons of fabulous lines (such as describing 1979 as "more of a table wine", rather than as a vintage year) and some really striking imagery: the sketch of Romana with a cracked clock for a face is imaginative (and illustrates the problems with time very well), and the shot at the very beginning, of Scaroth's ship taking off and exploding, is really nicely done. Scaroth himself is a suitably interesting villain -- both in visual and characterization terms; Julian Glover exudes sophisticated, charming villainy, and he's a joy to watch. Plus that first cliffhanger, where he rips off his fake human face to reveal the Jagaroth underneath, is certainly memorable (though it's not quite clear why he's doing so; maybe he needs to let his real face breathe).
Part two is even more wonderful. It's chock full of great dialogue ("I say, what a wonderful butler, he's so violent"; "Now, while we're here, why don't you and I find out how they're going to steal it and why. Or are you just in it for the thumping?"; "You're a beautiful woman, probably..."; "Duggan, why is it that every time I start to talk to someone, you knock him unconscious?"; and scores more), but it's also plotted very cleverly. The scene with the Doctor talking with Professor Kerensky is fascinating, and the idea of raising money by stealing the Mona Lisa and then selling seven different copies of it is marvelous. I also love how the Doctor and Duggan are having a conversation in Scarlioni/Scaroth's cellar while Romana is in the background investigating why one of the rooms isn't as big as it should be. It's all simply gorgeous.
And another great cliffhanger: the Doctor pops back to Renaissance Italy, only to find that Scarlioni is also there...