Take a look at LMMS (Linux Multi-Media Studio). This project begun as a clone of the popular windows software FL Studio. I have been using both a few years ago, and LMMS was still having a lot to catch up with. It has support for most of the basic plugins and features already present in FL Studio, which are VSTi plugins. Also, it supports the FL Studio file formats, although I'd recommend not to work on the same project files with both FL Studio and LMMS if you have the two of them installed, or at least backup before opening with the other editor.
Basically, if you are familiar with FL Studio, it has quite similar beat sequencer, playlist mixer and sample editor, so it should correspond to your second and third points.
As the LMMS site says, it has support for the following stuff:
- Song-Editor for composing songs
- A Beat+Bassline-Editor for creating beats and basslines
- An easy-to-use Piano-Roll for editing patterns and melodies
- An FX mixer with 64 FX channels and arbitrary number of effects allow unlimited mixing possibilities
- Many powerful instrument and effect plugins out of the box
- Full user-defined track-based automation and computer-controlled automation sources
- Compatible with many standards such as SoundFont2, VST(i), LADSPA, GUS Patches, and MIDI
- Import of MIDI files, Hydrogen project files and FL Studio ® project files
From the above said, I have not used any external MIDI device, so I cannot say if the USB support is available and properly working. Anyway, it seems it will certainly fit your first 3 requirements.