These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Takenobu Mitsuyoshi, composer and singer of many of Sega's most famous arcade tunes, performed a vocal version of the song "Conquista Ciela" from FORCE and MARZ. And it is glorious.
Even Better Sequel: Operation Moongate is already a solid game on its own right, but Oratorio Tangram improved so much on original game's formula and added more depth to the gameplay, it's no wonder the first sequel is a favorite among fans.
Game Breaker: Fei-Yen from the original game, in the hands of a remotely skilled player is near-unbeatable, especially after her Hyper Mode.
Viper-II's Homing Beam can be fired at point-blank range in Close-Combat mode when long-range weapons are not supposed to work. Considering it's massively overpowered and, well, homing, this can do a lot of damage.
Hell Is That Noise: Specineff has a special move called "Death Mode", in which he gets 13 seconds of invincibility and later instant-death to Specineff afterwards, if the user's life meter is lower than the opponents. You can hear the sound of someone's heart beating as the 13 seconds counts down, and after the death timer runs out, a scream can be heard from Specineff as if somebody's soul wailing from inside.
Polished Port: The Sega Ages 2500 Series Vol. 31 re-release of Operation Moongate for the PlayStation 2, which has fluid frame-rate and updated graphics (up until the HD re-release), crispper sound quality, and slew of extra modes like being able to play as Z-Gradt or fight with chibi-fied versions of the Virtuaroids. Unfortunately Western fans may never be able to play this version since it stayed in Japan.
XBLA port of Oratorio Tangram Ver.5.66 has optional HD visuals, online multi-player, tutorials for beginners, and the color-edit feature from the Japanese Dreamcast version that was implemented into Ver.5.66. The default control scheme works quite well with the Xbox 360 controller. It may also qualify as an Arcade Perfect Port as the Arcade Mode plays just like arcade version and there's virtually no loading times. The only major downside to the XBLA version is the lack of split-screen multi-player (although it does have system link support).
Sequelitis: Virtual-ON: FORCE, and eventually MARZ.
That One Boss: The massive VR Jaguarandi ever since Operation Moongate. In MARZ, you not only have to deal with him, but if you do well enough in the stages leading up to him, you'll have to fight two Jaguarandis at once.
While Jaguarandi is an immense pain in the arse in FORCE, the final battle with Ajim and Guerlain is quite possibly one of the most ruthless boss fights in the series, especially coupled with FORCE's more sluggish controls.
Ajim in Oratorio Tangram is an SNK BossMade of Iron, contrast to being playing him. He alone is a good enough reason never to win any time-out victories.
In MARZ, there's also the one encounter with the White Knight.
Deconstructed and parodied in Operation Moongate if you choose to continue after getting beaten into a pulp. Jaguarandi's armour has halved by then, and to boot, he shrinks to half his size. If you still can't beat him, and continue again, this repeats. Ad infinitum until he's so small he's practically invisible and one hit will kill him.
If you're playing the Windows version of Operation Moongate on newer operating system, however, chances are your game will crash thanks to a bug and you have restart your game from the beginning. This can be very rage-inducing if you lose to Jaguarandi unless you're Fei-Yen, which (strangely) the bug won't cause your game to crash.
That One Level: Operation Moongate's Bonus Boss level with Fei-Yen from the Japan-only Sega Ages PlayStation 2 port. She runs and runs and runs while firing, and you end up dying because you took more damage than she did and the time ran out.