True Legato and Portamento Sample Overuse
One of the most overused things in midi mockups are true legato and even worse: portamento patches. With many people using these patches overly extensively the result is very often a piece that sounds like chewing gum.
Be aware that these things are usually considered as bad taste amongst musicians. String players for instance would hardly every play a portamento and only do that in very specific individual transitions or when specifically requested to.
Brass players on the other hand very rarely play a real legato. Especially in ensemble lines there is most often a quite deliberate separation of the notes.
Legato is not the default playing technique for every player. The only section that quite often plays real legato is the strings and even then it is tried to keep the actual transitions rather short and tasteful.