Poor colour, mostly. Also, when I set it to reasonably high resolution, it's a lot better at picking up surface artifacts of the film than the picture in it.
I see you have some fluffies in the pic. The biggest problem is getting a clean scan - that involves cleaning the slide/negative, as well as the scanner. A flatbed scanner means you have to clean the glass AND the cover for the light in the lid (you usually only have to clean the scanner once per session). I wasted huge amounts of time cleaning and re-cleaning a slide before I realised that the dust was on the light cover in the lid
wrt colour, I switch off everything in the twain software that has anything to do with image enhancement - I prefer to do that myself. However, the stupid Canon software always switches on "Auto tone" everytime you start it. I found that this Auto tone give the scanned image a horrible red cast. Vuescan should bypass that, especially if you use the presets for the particular film you used (although on second thoughts, Ed Hamrick never developed his own drivers for these particular scanners, he uses the Canon drivers).
The speed on the 8800f is acceptable when compared with the equivalent Epson scanners (again, I can't comment on what the 4400f does). That speed is light years ahead of what my Epson Filmscan 200 could do - that was slow.
I do agree with Ponder, that is a nice pic!