I should begin by saying this is a personal review of DA and in no mean should it be taken as anything else
The GoodBy far the best two aspects of the expansion in my opinion are the hazardous enviroment planets and asteroid fields. These add a nice new twist on the gameplay and make it feel fresh.
The new ship designs are very nice and I find myself using template ships most of the time. They are cool looking and I cannot make any better designs myself, so I use templates.
New ship combat system feels better than the old one and it actually makes more sense now that every gun is handled independently.
I find Mega Events neat and they are just what they are very Mega and shake up the game like an earthquake.
The tech tree boxes have made the tech tree easier to navigate.
New galaxy creation menus are very easy to navigate and good overall.
Some gameplay tweaks are very nice IMO, like like limiting ship speeds via bigger engines and being able to use diplomacy less frequently.
AI seems bit wiser and provides real challenge even on tough difficulty level! And its getting better with patches, enough said?
The BadUI. I find UI the most dissapointing aspect of the DA. I admit being a bit of a UI zeatlot and find myself constantly irritated by even the slightest short comings of UI. Overall DA UI does its job and I have seen many games with far worse UI but there are many places where the UI just seems unfinished, you only have to open the rallypoint pop up menu to see what I mean.
Espionage system. This subject was discussed to the death on many topics during beta but developers shooted down the communitys ideas. Instead of implementing a deep/rich espionage system we got this over simplistic shallow espionage system which I would rather have as an option than hard coded feature in the game. Disable buildings or remove enemy agents, thats the espionage in nut shell.
Diplomacy. If my memory serves me correctly, improved diplomacy was on top 3# when the most wanted expansion pack wishes were asked in a poll. So whats new? Two threaties that make it easier to gain alliance with an AI opponent. Where is the more meaningful United Planets? The ability ask opponents to remove their SB from your influence zone and so on? I also must mention that the whole diplomacy trading lacks real logic, you must guess what makes the deals work since there is no indication what the AI values and how much.
Land combat. Ship combat has gotten lots of love with patches and now even more with the expansion pack, but how goes it on land combat side? Billions of citizens are given laser guns and send off to commit genozide on another planets. There can be all sort of explanations why this is so, but the end result is the same, Land Combat is over simplistic and boring. Where are the army forces that I remember were talked during development?
Game Mechanics. There are many just plain bad gameplay mechanism quirks that have carried over to DA from GC2. Here is a list of some game mechanic flaws that come to my mind:
1. The most obvious is ofcourse the absolute micromanagement hell of starbase constuction. There are people who say "why not use rallypoints" etc, but its a lame excuse in my opinion, since even if you use rally points it does not become much clearer. This because when the constuctor hits the SB and ask you "what would you like to build" you have no idea which SB is in question. Clearly the developers took another route with mining bases, why not implement same sytem on SB construction? Automaticly and neatly, no need to bother the player with trivial things like moving every single constructor to the right SB.
2. I find it akward as h*** that there is no hotkey for moving autopilot moves. This is something GC1 had but GC2 is missing, very weird.
3. Lack of borders. Yeah Yeah I know space is infinite and there are no borders in space, but this is a game. We humans define everything with borders like our countries, our houses, personal areas and the list goes on. I fail to see the logic why SD has to try and make GC2 unique this way. I think its a bad move as it goes against our very nature as territorial animals
The UglyDA is a pretty game except the utter ugliness of land combat!

If it was my game, I´d be ashamed of the land combat graphics, but thats just me.
Ship Combat videos. They are better in DA than in GC2 but I find the automatic camera quite bad overall. Either it jumps all over the place(cinematic) or zooms way too far away from the battle (top down). Ofcourse free camera solves this but consider the lazy folks

Formation choosing for fleets would have been nice add too.
There are literally tens maybe hundreds of new premade ships at your disposal with DA, but zero new portraits and civilization logos to be found. Somebody at graphics deparment had brain shortcut
Planet textures are blurry and ugly.
Verdict = 7.8pI admit that I had too high hopes for an expansion pack and that is the reason for such a long "The Bad" section. Hopefully this can be taken as constructive critism and developers consider these things when GC3 is on the drawing table.
Also it should be considered that this rather harsh verdict comes from a fellow who has probably played too much of GC2 alrdy. So its only natural that the most intense WOW just isnt there anymore. All in all I would say that the expansion pack is a must buy for a relatively new players of GC2. If you have beated GC2 to the death over and over again, DA probably wont bring back those sleepless nights like GC2 did.
Hope people can discuss their own views of DA and take my review as it is. Which is just my personal view of the expansion pack and nothing more