I don't understand how the two developments on Quinpool (not the one by the rotary) were rejected due to height limits but the Quinpool Towers and Atlantica Hotel are right there.

The plan has specific limits on height, based on impacts on surrounding area. Specifically, that site was limited to 20 storeys, to prevent excessive wind and shade on the commons (if you've ever walked across the commons in the winter, or gone skating, or played baseball or cricket there, you'll know how important this is). The developer (george Armoyan's company Armco) knew of these restrictions, and came up with a proposal for 22 storeys. Council said that was out of spec, and to redesign. Armoyan came back with the same plans, except even taller, 28 storeys. Council said hmm, that's okay for now. then a new Council said no, max 20 storeys. I think that's where this lovely local drama is so far. Basically, the developer is kind of a dick, old council was kind of a pushover, new council isn't.

I dont think you'll find anyone who says that a redevelopment at that site is a bad idea. but most people would agree that, since it is at a major intersection and will have impacts on the heavily used commons, the development should be a good one. This isn't some private property owner getting annoyed because a 6 storey building is going up somewhere on the same street as their 2 storey detached.