1. I love the new rules for team play in this expansion!
I like games where teams of two play against each other in general (particularly the Settlers of Catan Team Game, see my comment for the Settlers base game).
In 7 Wonders, many cards (or wonders) have effects for just your direct neighbors (for purchasing resources or even some direct bonuses with the Roma B wonder, the 1st stage of Great Wall B and Gambling Den and Gambling House from Cities). Or their strength varies dependent on what your neighbors build (most Guilds, Vineyard and Bazar, Mask symbols from Cities or on the Great Wall B, 2nd stage of Stonehenge B). When you let players choose the sides of their randomly selected wonders (as I do), the combined choice can also have interesting team play implications (particularly with Manneken Pis and Stonehenge).
All this makes it especially interesting, that one of your direct neighbors is your team partner!
The leaders Hatshepsut and Esteban should get considerably stronger in team play (maybe a little too strong, actually). The cards from base game and Cities that let you purchase resources from your partner at a reduced cost get considerably weaker.
The rules for team play don't seem to have much to do with the Cities expansion, at least at first glance. They might be used with just the base game as well. However, on a second thought, without Diplomacy Tokens, the two then guaranteed battles per age against only one opponent may be too all-or-nothing, I guess that'll play considerably differently (than with the Diplomacy tokens from Cities), probably worse (particularly with only 2 teams, but also with 3 or 4).
2. The possibility of an 8th player in this game which is especially suited for many players is very nice to have as well!
3. I like the new mask symbol mechanic. In contrast to the scientific symbols on 3 leaders in the Leaders expansion, the masks don't really make a scientific strategy stronger per se compared to the base game. They rather offer some counter against neighbors who mess up your strategy by going for green themselves.
4. I like that a) the possibility of Diplomacy tokens in games with enough players and b) the existence of the high military cards Militia, Mercenaries and Contingent provide a stronger incentive to extend a military lead over a neighbor with no or few military who doesn't seem especially interested in it, because a) you may want to be prepared to need to fight against the guy on the other side of your neighbor and b) the many shields on the black military cards can enable the neighbor to overtake your military with a single card, which may convince them to get into military after all (particularly if they can build it for free with Caligula and/or additionally profit from black cards due to Darius, Diocletian and/or Secret Society).
In the base game I often found it somewhat unsatisfying when somebody wins most or even all of his fights (and often the game as a result) with very little military just because he was lucky enough to be seated between two neighbor(s) that armed next to zero - maybe because they were busy focusing entirely on green cards or something else and/or had a good reason to assume they'd loose an arms race in the end anyway. This can be perfectly rational for them. However, the military victor(s) on the other end of the table (with 5+ players) may have had to invest much more so that it wasn't really worthwhile for them in the end. The peaceful players without any military are obviously the ones who have the highest incentive to play diplomacy cards, so there is a good possibility that the military guys compare directly at least in one fight, which makes the distribution of military points a little "fairer".
If you get all those juicy military points, you should at least have worked for them to keep things honest!
I also like that Cities somewhat weakens military overall, I found it a little too important before - at least in games with few players. In addition to a) the cards with dove symbols which reduce the number of points given out by military (particularly with few players) and make it a little more unpredictable, there are b) Militia, Mercenaries and Contingent which make it harder to secure a military lead and c) Sepulcher, Cenotaph and Mourners Guild. Only one card (Slave Market) helps military strategies.
5. I also like the new element of Debt tokens and interactive cards that steal money. 7 Wonders has lacked a mechanism to directly attack your opponents before (other than military) and I think it is good that this gap was closed. I particularly like that the money stealing cards attack all other players and not only the neighbors (or even worse, a player of your choice). Otherwise some players would be punished too much for sitting next to players who play aggressively.
1. Although I generally like that Diplomacy tokens weaken military and seem to make it a little "fairer", I don't really like that they introduce a considerable luck factor in games with few players when only 2 or even only 1 player remains that participates in conflicts. This is mostly an issue in games with 2-3 players, sometimes with 4 players and highly unlikely with 5 or even more players.
It is particularly a problem when the tokens don't come from a wonder (Byzantium, Great Wall B or Manneken Pis A), where you can at least try to plan for them in advance, but from black cards (Embassy, Consulate, Residence) or the leader Aspasia, whose existence is totally random (especially in a game with few players) and often can't be anticipated in time. That considerably increases the luck factor when you commit to a military strategy early (especially with leaders).
However, at least it weakens military where it used to be strongest, arguably too strong - in games with few players. One has to think of it as "in this 3 player game there is a good chance that dove symbols will be played even though there are none on the wonder boards, therefore military is not as dominating as in 3 player games without Cities and I should invest less in it". What still bothers me is the increased luck factor of gambling on it, but in games with three players, you have at least seen all the cards of an age before playing the last four hands, so you can try to postpone your decision of how much to invest in military until then. And you can obviously also remove a black dove card from play by building your wonder or discarding it for money (or the Babel tower when playing with Babel). Even the other guy who probably wins one fight and looses against you has an incentive to remove a dove card in ages II and III.
All in all, I think my positive point #4 outweighs this negative point #1, but I'd have preferred if it had done it in a way without the increased luck factor.
2. A (little, but totally avoidable and therefore annoying) minus of the expansion is that for some reason, 5 of the cards are not pictured nor even mentioned in the rule booklet (Gates of the City, Tabularium, Capital, Secret Society and Slave Market). This makes it considerably harder to get a good overview of the new black cards, particularly during a game.
This comment has grown long enough by now that I plan to convert it into a review sometime soon. I'll probably wait until I have played a few more games with Cities to support (or change) my thoughts, though.