The loss of item slots really does hurt Megas more than it seems, but I feel like this will become more apparent as the meta begins to define itself.
For example, choice band scizor actually has more attack than mega scizor which means if using scizor to one shot priority targets rather than as a set up sweeper, regular would be better. (In fact life orb scizor seems to have more attack, only reason you would use Mega is for increased bulk, no recoil, and the fact that after a swords dance, life orb advantage is negligible.) I'm almost certain life orb on blaziken does more damage too, the mega form basically took away some of his damage and threw that into his speed and bulk.
As far as mega walls, I think they won't end up as pure walls, due to lack of leftovers which really hurts more than it lets on. In the case of venusaur it might be trained 252 hp and 252 offense and just be used to tank hits that it can survive and return a KO or perhaps as an aggressive leech seeder for those wanting it more defensive. I feel like making it a pure offensive pokemon still leaves it open to getting beaten in terms of damage, yet making it a pure wall leaves it with little damage with a lack of lefties to mitigate the damage. Some people argue synthesis and giga drain make up for it in which case it does not in my opinion.
Leftovers is free 1/16 hp, which as the battle goes on will give a ton of hp for free. After 4 turns its already a free 25% of the pokemon's hp. And as a wall you expect to be out for longer periods of time. In the case of mega venusaur, It'll lose to some of the most common walls like Blissey and Skarmory due to lack of passive heal (by lose I don't mean he'll get KOed either, but rather they'll get the chance to set up entry hazards or whatever they wanted to do and run away and get more accomplished). In fact venusaur's offensive coverage is pretty weak so even against pokemon that can't beat it like ferrothorn, it can't kill them back before they set up entry hazards and switch to a suitable threat unless venusaur invests in offense.
Also on venusaur itself rather than a mega overall, it can't be a wall because in a wall stall battle, any wall with a reliable recovery move will outstall it just due to leftovers and more pp in the active heal move. Giga drain sounds good on paper until you realize if you don't deal damage with it, you don't heal with it either. Giga draining a wall won't give you any more heal than a leftovers.
