The HF S21 has an EVF; the the HF S20 and S200 do not.
Canon uses the limited bezel space for one dual-function button. While playing back video, it handles the wireless uploading; while shooting video, it lets you toggle between the Powered IS and the current image stabilization state. In playback, it allows you to downconvert HD video to standard definition in-camcorder and copy to an Eye-Fi SD card for wireless upload.
The low-resolution EVF is better than nothing--which is what the HF S20 and HF S200 offer--but pretty coarse for manual focus. However, between the focus-assist magnification and peaking for edge detection, it's relatively usable. Because of its location, the diopter is a bit annoying to set without poking your eye out; your eye and your finger need to be in the same place.
The battery recess is clearly designed to hold a larger battery. I suggest you budget $75-$100 for the higher-capacity BP-819 because the supplied 890mAh model is woefully underpowered.