Well only one, if you keep your phones long enough like 2 years or so, get a 920, otherwise 820 is wonderful, i love mine, fast & fluid; just what you would expect from a windows phone. I love it enought but i would like you to know this:
1. Low internal memory, 6GB user available, not at all enough if you play games/download videos.
But 64GB expandable storage is a big plus for music/movie lovers.
2. Need to take care of screen, it not A Gorilla Glass. Screen protectors really irritate outdoors, too many reflection, no matter how good you buy.
3. Battery life might be an issue if you are a heavy gamer otherwise gets me a full day with surfing/calling/chatting.
4. Camera is nice, but after seeing 920 everything seems bad in low light. (Not an issue, just jealous of 920 & even 720)
5. Horrible front camera. Usable only in bright light.
6. MicroSD card is a big pain to remove & insert. You need to take the battery out.
7. Color shells are really awesome and increase the love for the phone, and make it more funky.
8. Wireless charging optional, its an awesome feature, but you will need to buy a wireless charging shell and a charger, & for 820 + wireless charging shell + charger = 920.
(You will need to buy a charger & you will be stuck with one color forever)
9. Much better form factor than 920. (A major reason why i bought it, twice)
Need to ask any other specific thing, please feel free.