Writing music for specific orchestras/ensembles

Note: This is a hypothetical situation:

Say I was writing an orchestral piece, and I was planning to give it to my local orchestra. I know that the 1st clarinettist is brilliant, so I write an extended solo which I maybe wouldn't have done otherwise.

What do you think about writing music for a specific orchestra, who will hopefully be able to play it well? When doing it, do you think about the players themselves and their standard? Or do you just compose for orchestra/ensemble without having anyone in mind?

There is a time-honoured tradition of this, especially in the days of court orchestras. 

Haydn, for example, had an orchestra of virtuosi at his disposal for Prince Esterházy's entertainment, and he often made special use of individual players' strengths, especially earlier in his career.  When writing specifically for his patron the Prince, who played a bizarre, now-extinct instrument called the baryton, he was careful to give the Prince interesting things to play while staying within his limited technical abilities.  When he went to London, he was also well aware of the fine orchestra he was going to be writing for, and his final 12 symphonies show it.

When writing for a specific ensemble, I almost always take into consideration what I know to be their strengths, and almost more importantly, their weaknesses.  For example, when writing my Missa Brevis 4 vocibus (posted here), the commissioner advised me that his soprano section wasn't really capable of singing above F at the top of the treble staff, hence I only once wrote a G for them (having no other choice in that spot), but otherwise kept the range of the soprano part capped at F.  Though it was limiting, had I not done so, it would have caused problems for the very people who were paying me for the composition.  However, he had a stellar tenor section, including himself (and I knew his fine voice well), so I several times took the tenors up to A above the staff, and they performed admirably.

When not writing with a specific ensemble in mind, I'm freer with my expression, though I still usually take into consideration the technical abilities of the typical professional musician, unless I am writing something like a concerto, in which case the solo part is written with virtuosi in mind and is considerably more difficult.       

This is an interesting scenario. I've done both things (written for specific people, and written stuff without anyone in mind,) and I think that the most important thing is that if you're writing for specific people, they should know and you should tell them what you're doing to some degree. You should be very familiar with what repertoire they can play well and what's their overall technical level.

I've had mostly good experiences with this as people I've written things for trust me enough to let me do whatever I want, and it's worked out pretty well. However, you can't count on that being the case and it could as well be that they can put restrictions on what they want you to write, etc etc. If it's an outright paid commission, then sure it doesn't matter that much that you cater to their wishes since a job's a job, but in my experience I've always done things in a way where I have the freedom I need to do my thing first and foremost. However, I understand that may not be always possible or reasonable.

The best way to go about it, in my opinion, is to write for no specific person and write what you actually just want to hear. Then, after you've written the piece, see if there are any comments on possible changes or interpretations that you may be willing to compromise on. I think this gives off the best impression of you as composer since you are sure of your work but at the same time you are open for suggestions, just remember that you are the boss in the end, with all the responsibility that entails.