Keep in mind the term 'jailbroken' really only applies to iPhones.
That said, you should visit howardforums.com for info on Windows Mobile in general, in addition to learning about your new hardware and specific apps it can run. Yes, there is software you can install to turn it into a mobile hotspot. You also get the option to load a custom ROM to increase the phones performance. XDA forum is the place to visit for this info. WMexperts is another decent place to have a look around.
Which version of the HD2 do you have? You will only have ATT 3G coverage if your handset has the appropriate 3G (UMTS) frequency bands for ATT service. That means you need 850/1900 UMTS. AFAIK the two international versions are either 900/2100 or 850/2100 UMTS. Even the Telstra version won't help you in areas where you would need 1900 UMTS. T-Mobile version has 1700/2100, so EDGE will still work, but that seems like a pricey handset just to get EDGE.
I've never owned a jail broken phone before so I'm kinda new to all this stuff. I'm close to buying an unlocked and jailbroken HD2 for use with AT&T. Can anyone tell me what features I will not be able to use, where would I get apps, will I have 3g coverage, can I use this phone as a mobile hot-spot and ANY OTHER USEFUL INFO. i would greatly appreciate any knowledge you guys can share.