This seems to be happening on a project-by-project basis. Currently I am working on an ASP.NET 4.5 Web Application Project. Intellisense was fine one day but not the next. I am using a custom project template but that should make no difference. I tried editing the text editor settings so that Auto list members and Parameter information are both checked. I tried to reset your Visual Studio settings (Tools --> Import and export settings --> Reset all settings) and I also tried the solution in the accepted answer here:
Has anyone got any insight or suggestions? Pending any answers, I've submitted a Connect bug report: https://connect.microsoft.com/VisualStudio/feedback/details/766891/intellisense-broken-in-vs-2012-rtm