At least 10 death pits, preferably inside the spawn area. /sarcasm
But for real; certain game modes are more popular than others. TFC (TF2's predecessor) was based around CTF. The game was fast paced and the skill curve was as tight as Quake's. (TFC, afterall was an adaptation of quake CTF, on the quake engine (taken and modified by Valve into the Gld Source); an adaptation with class based team play). Every game mode was a variant of CTF, whether it was classic CTF, or invade mode CTF (take the flag from your base to the enemy capture location) or gather the flag (collect all 4 flags at your teams capture location) or bag handler (aka murder ball) where you have to hold the flag for the longest (often played with 4 teams, but sometimes with the typical RED/BLUE) and finally Attack/Defend CTF.
TF2's evolution left it with much slower gameplay and so CTF was left as a kind of redundant past time for Team Fortress. Many of the classic map designs no longer work as effectively as they once did, in both layout design and the with regards to the new class/weapon mechanics.
Fundementally players are drawn to new and interesting things so i feel like listing existing mechanics that have been the most successful only tells half the story. Payload is an adaptation of "escort" from TFC which involved escorting 1 player to an end point of a map where the defending team was limited to snipers. It has probably been the single most successful game mode of them all because of the many gameplay variables that suit many classes and weapon loadouts; the mechanic of the payload keeps large quantities of players together creating a peak of gameplay-combat throughout the map's length/timer. Secondly King of the Hill sees a significant amount of play, in a similar manner the single objective in the centre of the map focuses gameplay and keeps players in combat for the most amount of time (less travelling and capturing objectives away from the fighting).
The intense fighting that makes certain maps and game modes more exciting than the alternatives (if maybe a little more repetitive) also makes some maps popular despite the fact that their balance is skewed. CTF 2fort has a reputation for being unbeatable and constantly stalemate-prone, but players are drawn towards it anyway because the peak of combat is fairly consistant within the no-mans land between the bases.
In terms of what makes a map successful you can be looking at many variables that can contribute to a maps success but usually new things will peak interest in any custom map design. Either aesthetically or as a new game-mode concept such as the sequential capture map cp_steel. New mechanics like lifts (pl_hightower) or dynamic bridges (pl_thundermountain) can all contribute to a unique experience that attracts players to specific maps. This can include "death pits", otherwise known as "environmental hazards" to level designers/mappers; which include literal pits of death, or trains/crushers/scripted explosions.
Gimmicks will attract players to your map, though the more reasonable the gimmick, the more successfully it is incorporated into the TF2 universe (aka not mario-kart), also increases it's general success within the hard core TF2 community and subsequently the level of attention it garners from TF2 developers that might eventually consider it for official material.
P.S. Koth and Arena are the easiest because they are generally smaller maps and obviously take much less time to construct and run an artpass on; but also because of their simplicity makes play-testing more straight forward and the game entities are not as complicated as other larger game mode experiences like payload which requires a greater understanding of point-entities.