I'm speaking as someone who tends to be in the low-ish gold tier, but has approximately 1000 games played as Veigar, my previous favorite champion, and has played him competitively.
Let's break down the changes that were made to him this patch.
1.) Baleful Strike
This genuinely has positive and negative effects, though in my experience playing him with the changes, it's significantly more negative than positive. It's now a skill shot that hits two targets, it has more range, and it has a shorter cooldown early and a higher cooldown late. There are some obvious benefits to this - you can hit people farther away, you can set up skill-shots behind you while backing off, etc. You can also hit two targets, the most significant aspect of which is that you can potentially kill two minions with one Q. As mana-hungry champion early, this can be a major boon. However, the downsides far outweigh the upsides. You no longer have reliable poke, especially against champions with good mobility. The fact that it costs so much mana limits how much Veigar can take advantage of the reduced early cooldown, while later in the game his reliable damage skill in sustained fights has a 25% longer cooldown. The highly-touted "kill two minions" benefit only works if there are two minions in a line that can be killed, while I found more often in lane that I had to unwillingly damage a minion I didn't want to to pick up a kill behind it or vice versa, and I missed a lot of minions I'd like to pick up because there were two or more others in the way and my lane opponent wasn't a complete moron. Furthermore, with certain minions being worth more than others in terms of AP, not being able to directly choose your targets hurts even more. Picking up two regular minions but losing a cannon minion because I couldn't get a shot at it cancels out in terms of extra AP. Later in the game, when I could one-hit minions, this was less of a big deal, but by the time that happens you're out of laning phase and are spending a lot less time farming. In teamfights, the increased range lets you play a little safer, but again you now have a skillshot that can be dodged. In particular, people who now flash away with low health after you cast at them aren't still going to be hit, which really hurts Veig's kill combo against champs tanky enough to survive a W-Ult combo.
2.) Dark Matter
It's cheaper and has a shorter cooldown at higher levels. No complaints there.
3.) Event Horizon
It's now cheaper at high levels, has a slightly shorter cooldown, a slightly longer range, and takes 0.75 seconds to kick in. This skill was Veigar's bread and butter, and all the minor nice changes above aside, this change absolutely cripples him. Obviously the intended plan was to get rid of Veigar's stunned from full to dead terror combo, and I'll freely agree that this is not an unreasonable thing to introduce some counterplay options to. But having played Veigar in the new patch, I have literally had people walk out of the field after I caught them in the middle. Or done any of the things the could already do if you didn't catch them right with it before - flash out, shaco-jump out, cast on you from inside it, whatever. The point is, as a glass cannon champion with a high-damage skill that takes 1.25s to hit, Veigar was completely reliant on his stun to be able to do two things - hit with Dark Matter, and safely enter cast range to use his other skills. Basically, he can't do either now. The only times I was able to hit with my stun with any kind of reliability was when casting on people who were already stunned or snared, or when casting into the confusion of a teamfight. And the latter becomes less reliable the better your opponents are.
But the other problem is that Veigar's stun was a powerful defensive tool. Veig has always been a pure glass cannon, and most assassins can kill him in the time it takes his new stun field to kick in, even if they don't bother to dodge it. Without being able to use it defensively and blow up assassins before they could blow him up, Veigar now has no counterplay options against assassins, especially with the nerf to his ult and the decreased reliability of his other two skills making tankier Veigar builds that are still useful unfeasible. This is actually the biggest nerf I found, because without any defensive tools, you're way too easy to kill to make being a mid-to-long-range sustained Meteor-and-skill-shot poker a viable new play option.
As to the increased range, I'm rarely casting stun field at full range except to either initiate a fight or catch a fleeing opponent. In either case, the 0.75s reaction time the opponents get vastly outweighs the benefits of an extra 50 range.
4.) Primordial Blast
It's cheaper, it has a 10s reduced recharge at all ranks, and it had it's AP ratio reduced by 0.2. The cheaper bit is always nice, although with Veig's passive, mana problems later in the game usually don't happen. The 10s reduced recharge is, hands down, the thing I found to be the most beneficial. The 0.2 AP reduction, though, hurts a lot. Presumably it's to make up for all the extra AP Veigar will be getting from his new Q, but it's nowhere close to enough to offset this nerf (and as described above, while I definitely got more AP with my Q, the tons of extra AP theory doesn't quite pan out for several reasons). Most of Veig's AP still comes from his gear, and especially with DFG just being removed, nerfing the damage on his big bursty skill really, really hurts him.
In conclusion, Veigar has all but lost his only defensive tool, his only way to initiate safely, his only way to close gaps against a fleeing opponent, and his only way to reliably hit his W. His Q has gained strength in some ways, but at the cost of his ability to consistently poke, use it to kill minions he doesn't have a convenient shot on, and a huge hit to his sustained damage in fights. His R, while he can cast it a bit more often, does a lot less damage and doesn't do a whole lot by itself except against already very squishy targets (many of whom do have single-target, instant stuns that they can use to kill him now). In the opinion of someone who is not a pro league player, but is a pretty experienced Veigar player, his new kit takes out all of his strengths and gives him almost nothing to compensate.
As much as I love Veigar, I know better than anybody how hard it can be to counter him if he's good, and will freely admit that some additional counterplay options against him would be nice. Something like a 0.25s delay in his stun to give players some reaction time without making it unhittable might be a reasonable possibility. Or making the interior of the field slow until the stun kicks in. Or making the entire circle stun anyone who's still inside it after the delay. Or making Veigar a lot more inherently tanky. Or dumping his W in favor of a survivability-based skill and moving the damage onto a DOT inside the stun field, like Malz's pool. There's lots of options, but these changes took away pretty much all of the strengths he had without giving him any new strengths to work with.
single 
mana-spamming 
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feels?
