Super Street Fighter IV with online multiplayer... that's all I needed to know.
As soon as the online multiplayer feature was announced, the decision was made for me to sell whatever I have to and do however much computer sidework is required to get one of these at launch. This is the ONLY feature that has influenced my purchase decision. I already have a DS Lite and have been getting a lot of use out of it lately reliving classic RPGs. I would have been happy with my DS Lite for years to come at the rate I am going with these games, but... the thought of playing SSFIV on the go without having to carry an XBox 360 is just too much temptation to resist. This also saves me the indignity of having to buy an iPad + controller + SFIV whenever all those pieces are finally available on the iOS side of things. Thank you Capcom!
spot and street pass are going to be fun too. People buy the DS stuff, always have, so having someone pass me and me getting points and prizes for just walking the streets, the mall, my job! Yeah that is a huge awesome thing for me