This review contains spoilers, click expand to view. Game started with a good idea, but turned out, that developers don't know, how to create a really good game on it.
1. Story. It's looks like the developers just took a little pieces from other games. Beginning of Portal 2, jokes about cake (which spelled one or two times during the game, which ruins player anticipation 'when I'll get it?'), even some chambers from Portal 2 included in a game (but this is more good than bad, because of player thinks 'aha! I'm played this level with portal gun, now I can solve it with a gel gun!'). Overall, looks like story added just because there is must be story, because playing game with speakers turned off doesn't affect the gameplay. But worst of all is ending. Both of them, because developers just put slightly modificated 'bad ending' from Portal and ending from Portal 2. The only character (exclude player, which model just slightly reskinned Chell) is a Core, but it's pretends to be not just a core, but replace GLaDOS by robotic modifications of voice and overall behavior. Remember Wheatley from Portal 2? He has normal human-like voice without any modifications, and felt like a human in a core body. But Nigel not felt like human. During the game we see him via screens (and in last missions as a watching core), and always hear him through speakerphone.
2. Music. Abarax, Harry101UK, and Christopher McEvoy made a good work with a soundtrack, it's very atmospheric and using loudspeakers in level design as music distributors is fresh idea.
3. Gameplay and levels. At a beginning, game gives a good impression by detailed levels, like sleeping room with collapsing items, and transiting tubes, when you see a decorations between levels. But then developer just said 'nah' and stopped doing such things. Level design is not good, but not bad. But there is logical errors, when old Aperture levels filled with Discouragement Redirection Cubes, Hard Light Bridges and Excursion Funnels, which just does not exists in time of old Aperture. Lack of really new mechanics, like Adhesion gel (scrapped gel from Portal 2, which can change player's gravity vector) or Reflection gel (not used in Portal 2, can reflect lasers). Unchanged aerial control, which makes controlling character in air very frustrating. Also there is bad 'bug', when gel flies too slow, and when spraying orange gel in front of you, you can accidentally step on not covered by gel surface and lost your momentum because of slow-moving gel just not registered yet. And this bug mostly shows itself in a time-trial level (which can be skipped anyway). The levels designed that way, that when you are missing the platforms, you in most cases fall into water and die, beginning again at checkpoint. And that's not fun. All puzzles are very easy, and I'm stuck for a long time only in one chamber, but most difficult in solving them is handling with controls on gel.
4. Post release. In first release of game there just no subtitles at all, but they are added to game with next updates in a few days. But developer commentary still don't have subtitles, which makes developer commentary impossible to read for non-English speaking players.
This game would not have received such bad review, if not the last thing.
5. Pay distribution. This mod is paid, and if it's paid, you are expecting a good quality product. But we receive not very good mod (which requires original Portal 2 in addition, it's not even a standalone game!), with lack of new mechanics, short levels and poor story. If Aperture Tag was a free mod, all that shortcomings will be written off to 'nah, that's a free mod, it's can't be perfect'.
This review contains spoilers, click expand to view.