It would be very nice to have the capability of having hotspots on the panorama. I would even settle to use the existing highlight areas with a URL linking to websites or other synths.

This reminds me of The.Rev's request for hotlinking. You can lend him support here: 
http://photosynth.net/discussion.aspx?cat=01b6f15f-42eb-49cb-a221-ed56615e1c47&dis=bad52d1f-5215-4de8-ab50-8c95ca35bceb

would be good idea linking a hotspot to other syths or images on flickr or facebook

I support this idea.

Linking a panorama to a synth would be great, and linking panorama to panorama would give users the ability to 'map' more complex spaces. hyperlinking to other sites, tagging people on facebook via panoramas and other hot-spot activity would increase user interest, and help to get away from the feeling of being inside a wallpapered goldfish bowl that the panorama often give me.

Regarding linking synths to other synths, panoramas to other panoramas, or from either to the other: 

It has long been a planned feature: http://getsatisfaction.com/livelabs/topics/linking_synths

True, it has been delayed: 
http://photosynth.net/discussion.aspx?cat=01b6f15f-42eb-49cb-a221-ed56615e1c47&dis=8aedd930-ad71-4dec-a20f-292bbb64bb24

However the Photosynth team is still interested (note the second to last paragraph): 
http://webapps.stackexchange.com/questions/10154/photosynth-has-turned-from-a-3d-photo-app-to-a-panorama-sharing-website/10157#10157

See Bing's announced Read/Write World project for more information on what the team is working on. You can find links to six full length video presentations from Blaise Aguera y Arcas and Avi Bar Zeev as well as two quick demos by David Gedye on vimeo at this link: 
http://bit.ly/readwriteworld

I support this and Memoryoke's ideas. It will be nice to tag people on facebook!