Axton has insane DPS and can throw grenades and get like 75% movement speed and a 40% damage increase just when he goes down alone, and can be revived by his turret. Salvador, while I don't know his skills exactly, is also made for survivability.
The Mechromancer is good if you play for hours, or are a high level (but that's not saying much as everyone gets amazing as high levels) and generally has the 3rd, maybe 4th, best DPS until her Anarchy gets high, and even then when she goes down her DPS drops rapidly, unlike Axton and Salv who actually get increased DPS when down, and when she dies she has to start back over again.
Maya and Zero has underaverage suvivability, but the Mechromancer is still nowhere near as OP as people make out, especially as Axton and Salvador can be DPS and tank characters too, without having to worry about going down like her.
Edit:
Though Blood Soaked Shields has changed, it now has 5 ranks, so she has suddenly gotten a lot better.