I personally own a toshiba tablet pc..
My list too look for
1) Processor: Pentium M or those desktop type M chips are the big sellers, personally I prefer Pentium M's there lighter on battery life, usually smaller in size and dont heat up nearly as much and arent as loud.
2) Screens: 15,15.4, 12.1, 17 ... what do you go for? Well look at the resolution for sure.. dont go for a 15" screen with a max res of 1024 .. trust me it looks bad

look for what you would prefer and most brands are switching over to that same type of material that sony uses on vaio notebooks that "XBrite" stuff.. makes it look clear but with a bit more glare
3) Memory speeds: Some still use 2100 most use 2700 a few use 3200.. look at what you prefer of course if u know desktops you obviously know the differences
4) Graphics: some will come with built in video.. please dont buy these.. please dont buy "shared video" or "integrated video" they will trick you by saying its an ATI, but really its an ATI "SHARING" memory from the mainboard and sucks hard.. Get ones that are "Dedicated" radeons or "Dedicated" geforces
5) Harddrive: Size is of course a factor, but then speeds a big thing.. question hard.. 5400RPM, 4200RPM, 7200RPM.. ? What is it?
6) Battery Life: Big BIG! factor .. its a notebook of course you need decent battery life.. I dont care if its sitting on your desk 75% of the time.. it comes in handy in the most peculiar places (watching tv on the couch while doing work is great

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7) The others.. CD-RW/DVD-RW/Wireless Builtin/Modem/etc etc etc
i do want to double reccomend a laptop cooler if you do go for a laptop that has some nasty probelms with warming your hands too much, i have on for my tablet in laptop mode.. works great.. feels cooler then if ti were off (targus chill pad)