Reach and Pounce are usually associated with two different styles of melee combat. If you have one, you don't need the other, but having both is nice too.
Flight is important to have after a certain level, but can be difficult to obtain. Either get your caster to give it to you, wait until you can afford flying boots, be a race that has wings, be a Mutation Warrior Fighter, etc. It all depends on your build.
All three of those things are pretty important. If you are in a party though you might be able to have reach or fly covered by a cater which could ease the load on your build.
Reach is important because of AOPs, and being able to fight larger creatures on a more equal footing. Attacking equivalently sized creatures from further away is nice too.
Pounce is quite the game changer. Being able to full attack starting from round one puts a lot of pressure on the opposing team. If you can charge in and take out an enemy in the first round, your party is likely to be in a good place. Especially if you can get the caster in the back or something.
Fly is important because then you can engage other flying creatures. Theoretically, a flying creature with a ranged attack can just kite you to death. You should always have a ranged weapon, but you probably haven't really invested in being good at it, so fly let's you do your melee thang on those pesky airborne creatures. There is also the utility aspect of flying that allows you to do stuff you normally wouldn't.
Ponce is the most important, as it will greatly increase your action economy.
Fly is nice to have too, specialy against fliying oponents. Against groudn-bound ones, it's good to have, but you'll probably be charging towards them, so it's not that important. It can probably wait till the mid levels.
Reach gains importance as you level up and your oponents start to be of bigger sizes, hence puting you at a disadvantage if your reach is shorter than theirs. But I wouldn't call it that important. It's also good to threaten more squares and thus take more attacks of opportunity. However, since you defined yourself as a BSF, it'd probably just be a nice bonus, not your character's focus.
Without fly anyone with some means of getting up into the air and do some ranged attacks is game over for you - you can't hurt them, they can hurt you.
Reach can do some funny stuff - you can save yourself some tumbling against bigger creatures that have reach themselves, that's already been said, but against a 5ft rech opponent..grab a reach weapon with brace and set up. If you can hit that unlucky fellow charging at you will suffer a hit with double damage and then an attack of opportunity to boot. With a level 1 character you can fit in 3 rounds worth of damage when it's not even your turn.
Some means of Full Attacking every round is critical to a martial character looking to be useful at higher levels.
Reach is handy but less critical than the above.
Flight- by some means- becomes a requirement as well. There's a reason people joke about high level D&D/PF being a super hero game, everybody's flying. [Well, that and they can survive falling from orbit.]
Pounce is really good if you can afford it at low-mid level, because when you start to fight versus giant enemies with reach and special abilities like grab, trip and so on, if you don't have enough CMD to resist, you could just die or be incapacitated in your charge.
Reach is awesome since start, because you don't have to stand just in front of the enemy to hit him, something that will give you enough breath and good positioning.
Fly is good, but there are many ways to get that and there no many flying encounters...
|Kurald Galain RPG Superstar 2009 Top 32|
Flight is great because it lets you bypass all kinds of annoying terrain on the battle map; plus, quite a lot of flight abilities give you a sizeable speed boost.
Reach and pounce are both very useful in getting a full attack off. Once you have iteratives (or TWF or Haste), then attacking twice or thrice per round is simply much better than attacking once.
Reach is also the easiest to get, either with a reach weapon or with the Lunge feat.
If you find yourself in an aerial encounter and you don't have a strong ranged attack, then flight is 100% non-negotiable mandatory. Because these encounters are rare at low-mid levels you can often get away with just carrying a potion of fly for the odd occasion. As you gain levels, you should look at a more reliable form of flight. I'd get serious about it somewhere around level 10.
Pounce and Reach are not strictly necessary, but they are very good abilities that are well worth the time and effort grabbing them.