Mercury is usually given as one of the weaker if not the weakest senshi, aside from Chibi-Moon. Her powers, especially in the earlier series are mostly defensive: "Shabon Spray (Fog)" "Shabon Spray Freezing". She is the group strategist in the manga and the anime, using information from her mini-computer to help find weak spots in the enemy ranksm and is often the one who stays calm in a crisis, in the first season finale she smacks Moon across the face to stop her from being hysterical.
In the manga she also has a power where she basically creates a bubble of hyperspace and can protect the other senshi or transport them into a safe area.
In later series of the manga and anime she gets less defensive powers of "Shine Aqua Illusion" and "Aqua Rhapsody", but in PGSM she actually starts with attacks that are offensive, "Aqua Mist" and "Shine Aqua Illusion" in quick succession.
In PGSM she seems to have actual attack powers, "Aqua Mist," and "Shiny Aqua Illusion," and has also manifested the ability to manipulate water into an pretty cool looking ice sword, which can give Kunzite a run for his money. She, however, doesn't have the mini-computer, and there doesn't seem to be much strategy involved in the senshi battles.
In every incarnation she looks pretty much the same as a senshi, blue hair, blue and white uniform, three earrings in each ear. In PGSM she, as all the other senshi have cell phones which have the ability to disguise them, which only Usagi has in the manga and anime.
In PGSM the only way she's really weaker is her confidence, or lack thereof, which earns her targeting by Kunzite to be turned to the Dark Kingdom's side.