HAARP was my first serious TF2 map. It uses the special "attack and defend ctf" gamemode, in which BLU team is on offense and capture control points by delivering their flag to them. HAARP is divided into 3 sections, each with 2 control points for BLU to capture. If blue wins the first section, they move on to the second, and so on.
The downside is that the A/D CTF gamemode is a little underwhelming; it's not as solid as the standard A/D Capture Point gamemode in TF2. There are a variety of problems, like how the flow can be broken up by a team forgetting their flag at spawn, or Blue taking control of the last control point and awkwardly fighting for a minute before they can run the flag up. However, I think HAARP makes the most of it. The map isn't as streamlined as Capture Point maps, but in terms of the fun you can have it's right up there.
The most important designs going into HAARP are the layouts of the capture points, which help make HAARP feel so different from other A/D CTF maps. Instead of being out in the open, the CPs are sheltered in small rooms. This makes it relatively easy for the flag carrier to get near the CP, but difficult for them to pull off the capture itself. In addition, RED players can block BLU from capturing by standing on the CP. All of this ensures that you get drawn out fights for each point, and awkward out-of-the-blue captures are minimized. To the best of my knowledge, no other TF2 map has capture points designed this way, so they are unique to HAARP.
The map's theme is also unique in the world of TF2. It's inspired by the conspiracy theories surrounding the real-world HAARP. Of course, the map doesn't resemble the real HAARP (a flat Alaskan tundra wouldn't be terribly interesting). I combined the HAARP aesthetic with a cool cliff-side secret base. The map uses tricks with the Source Engine's 3D skyboxes to let players look out from one end of the map and see other far-off areas along the cliff-side, without suffering any performance loss.