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.
Demonic Spiders: The scavenger droids are small, flying, hard to hit, interface-screwing, and invading of personal space. Their primary weapon is some kind of lightning beam with an impressive range that as well as being difficult to dodge also drains shields and health outrageously fast (especially if faced in pairs or larger groups). Also, their dying action in virtually every single instance is to do a kamikaze attack which, no matter how much you shoot it, cannot be stopped and is very hard to dodge.
Boss: Blast! I really hate those scavs!
Droid dispensers are extremely annoying. They aren't enemies in and of themselves, but are Mook Makers that are immune to damage. The only way to destroy them is to get up to them and plant demolition charges. They aren't that bad when deploying B1 Battle Droids, but Super Battle Droid and Droideka dispensers can be a nightmare to deal with if you don't take care of them quickly.
Geonosian Elites. To the player they're really only a nuisance, but they're incredibly lethal to your squadmates. During the Geonosian ambush on Kashyyk, you really have no choice but to let your squadmates "die" and avenge their sacrifice single-handedly.
It's Short, so It Sucks : Republic Commando's short single-player campaign ranging from eight to ten hours was a common target for criticism.
Seeing the scav droids drilling through your helmet in first-person.
Being swarmed by large numbers of Geonosian warriors.
The first few minutes aboard the Prosecutor can be rather frightening.
And also a scene later in the mission: the team has to escape into a garbage chute and find themselves in the darkness, so they have to turn night vision on... and realize they are between a pile of Clone Trooper corpses.