Paul, Unfortunately Flickr has messed around with the photos and the original size images are not available and the EXIF, while available in Flickr, is not still embedded within the images themselves. Still, I've had a look, tedious though it is to work with Flickr, and here's what I see....
For the shots of the flowers your shutter speed is in the perfect zone for mirror slap to affect sharpness. I notice you did not use the self timer and you did not use mirror lockup. Did you at least use a remote shutter release or did you actually touch the camera at the moment you took the shot? I'm assuming you used a tripod, but did you? The shot from the 50/1.8 looks dreadful, even at f/4. It may be misfocus. It may be shake. It may be both. The shot from the short zoom looks much better. In fact, even though shooting at 135mm vs 50mm and 37mm (possibly odd choices for a direct comparison) the shot from the long lens looks pretty OK, not like a razor, but decent. However, see my next remarks.
In addition I noticed that you are using neutral picture style with zero sharpening, or at least that's what Flickr says. That will produce soft results. Pictures need to be sharpened somewhere along the line, whether in camera or on a computer. If you shoot with neutral picture style and no sharpening it is expected that you will edit the images later.
Regarding the outdoor scenes, shot with Standard picture style and sharpening at 6 or 3, they look OK, but with Flickr having manhandled the files I wouldn't put much stock in what is there to be seen. In the bird feeder shot I notice you used AI Focus. That's seldom a good choice. I think you may have got away with it this time, but even Canon caution that it may not be the wisest choice.
I think all in all the evidence provided is not that conclusive. On the face of it I suspect your new lens might be quite OK. It's the 50/1.8 you should be looking at, or the approach you took to taking those shots. You also really need to think through your workflow. If you want great shots straight out of camera then you need to set the camera up to accomplish that. Shooting with neutral picture style and sharpening at 0 is not the right way to do that. But it's not just what you do in camera that matters. It's what you do afterwards as well. Having Flickr screw around with image resizing and stripping EXIF out is not my idea of smart workflow. If you're OK with that then that's fine, but if you leave control of image resizing to Flickr then expect the results to reflect the effort you put into the process. i.e. none.