Easy - Olympus has hit the IQ wall.
I started with an E-1 then graduated to an E-30. At the time, I was living overseas and doing a lot of travelling so I chose Olympus digital because I had a focal length range from 24mm to 400mm at f/3.5 max in two excellent zoom lenses. For someone like me who also enjoys canoeing, backpacking, etc. it has been an ideal combination.
I am making (and regularly selling) fine art prints of natural landscapes made with Olympus equipment up to 17" on an Epson Pro 3880. They produce beautiful prints that have often been complimented for their "watercolour-like" beauty which is not an insult to me as a photographer, but rather an observation of the way Olympus sensors render tones and detail.
However, I also recognize that to do the work I am more used to, along the lines of my old Pentax 67 and 4x5 Wood Field Camera, Olympus is far from mark. The 4:3s sensor just isn't large enough to pack in enough pixels to extract the level of detail nature photographers and landscape artists are looking for. Resolution is not the culprit as much as noise is. It's unavoidable. It's like people - the more people you cram into a smaller area, the more noise they are going to make!
I've hit the wall with Olympus and with the E-5 still at 12MP (ideal for a great many applications up to a double-page spread), it seems Olympus has also hit the wall in sensor design. A shame, really, as their glass is absolutely fabulous.